For babies you have to be very careful and gentle. Their food (milk is food) must be very bland and weak. This conflicts too often with their need for nutrition. Their guts haven't developed properly and are easily pushed into illnesses. It seems the main problem is establishing the creating the correct gut flora (the blend of bugs). They aren't born with that. It has to develope somehow, and that is bloody difficult.

I tend to believe the best milk substitutes are Wombaroo products. There is a variety to select from according to the type of animal and it's stage of development.

An alternative to Wombaroo is Biolac which is for all animals and has 2 or 3 grades according to their stage of development. This broad spread of animals that it can cater for implies that it can't be as well taylored for the specific types of animal. I don't know, but one thing it does have over Womaroo apparently is it's content of pre-biotics. I think these are things which can be used by the animal for the creation of more complex things. Dunno, but they are basic and important.

Other than that there are the lactose free formulae, which I feel could be technically inferior, but the big thing is that they can be easier for the animals system to handle, and this has its own important advantages (avoiding complications). Divetelact, Digestelac, etc. There is no recognition of different animals which have differing needs, and the final mixture is made with much less powder. It would seem to be weaker, if nothing else. But then again, this is biology. Who knows what really goes on?

Getting the baby animal to thrive on these substitues for their mother's milk is difficult. I always start off by giving a fluid which does not need any digestion at all. Vy-Trate or Lectade place almost no work on the gut and contain all the nutrients and correctly balanced salts to sustain life. The next day we start adding the appropriate Wombaroo milk to this, and over the next couple of days increase the strength of that blend. If the animal passes diarea (you should be allowed to spell it like that) then go back to the Lectade or Vy-Trate. I expect you can use this for many days to let the gut clear out and recover from disorders.

There are some tricks you can use to deal with sensitive guts. Small possums seem to improve when you make them some eucalyptus "tea". I guess one reason this tea works because eucalyptus is such a harsh substance and any bugs that survive in it would probably be dependant on it to protect them from competing bugs. Some yoghurts also have the right breed of bugs in them. Give them a taste of that.
Think about other ways of introducing the right bugs. Some of the correct bacteria for wallabies live in the soil. Put a bit of grass roots in the bag with small wallabies.
Another sure fire way is to feed them some poo from an adult. It must be fresh though. The bugs would quickly start to die and be replaced by bad ones the longer the poo is out in the air and cold. Protexin is a powder specifically for starting off the gut flora.

Gut flora is one of the most important needs of an animal. Another is it's immune response. And, of course, the two interact, as with just about everything in biology.

Gamma-G is a product specifically to address suppressed or absent immune systems. Young marsupials have no real immune system while they are furless. Use this a lot, especially when there is some form of stress on the animal.

What ever you try, the old problem is always with you - anything could upset that sensitive gut. When that happens it nearly always causes diarea, and you often have to fall back on harmless fluids, keeping it up for a day after you have seen the last of the diarea, and start all over again trying to get them to take milk substitues.

A vet is the best source of specific advice when diarea happens. Often you will have to use antibiotics to kill of gut bugs. Afterwards you have to start from scratch again to try to develop the correct gut flora.

Hang in there. Look forward to the time when you can get the animal on solid foods. For some reason they just take off at this stage, and it's plain sailing afterwards.

When it is time to introduce solid foods, use some bland, soft stuff. Be careful of contamination. You don't want to upset the gut again. Baby foods are bland and mushy, and also clean. Admittedly they are junk food for wildlife. Too highly processed and concentrated. But what else is there? Mash up their proper bush foods? After this you can slowly build up to stronger foods - bit's of fruit and meusli. Fruit is junk food too. It's unnatural and much less nutritious for wildlife. If you can't discover anything better ....

Don't confuse what is natural.
People everywhere are always falling for this myth that things from nature are natural. Well, they are, but NOT when you put them with something else - like the delicate gut of a small marsupial - they are emphatically unnatural. This sort of thinking belongs in adds with bimbos extolling the virtues of cosmetics. Ricin, the worlds most potent poison is "natural". You can find youself in real trouble in the world of bush tucker, and the world of substituting for it. The wrong type of food in the gut can cause a population explosion of harmful bacteria who may produce toxins or something else, like too much gas and the animal litterally inflates slowly and ruptures it's gut. Small animals seem as if they haven't yet learned to fart and relieve that gas buildup. They soon develope that skill though. Jees, you can't take them anywhere.

Here are my opinions, at the moment, of what you can give to native animals at the age when they start foraging for themselves:

Possums: They are omnivores. Varied diet - gum leaves, meusli, fruit, ..?
Wallabies: Herbivors. Use meusli, fruit, grass,
Bettongs: (richer foods) nuts, avocado, insect protien?, meusli,
Bandicoots (richer foods, like bettongs but more insect protien) meal worms too (from pet shops), ordinary worms and insects, bits of mince.
Wombats: They graze on tough, fibrous native grasses. Use meusli, grass, more grass,
Devils: Carion eaters. Give them road kills, your fingers, raw eggs, meat, bones, calcium supplement.
Quolls: Carnivores. Same stuff as for devils?

Note: the grass mentioned is native grass. That mushy european stuff everywhere often causes a pretty harmless bout of diarea (stuff the proper spelling again) and it must surely have an abnormally high water content. Use it if you can't get native grass from a roadside or the bush.

IMPORTANT:
I haven't done my homework on that information. I'll check up later. But that stuff applies for animals that have developed from the vulnerable baby stage.

Again, Wombaroo caters for special needs. Insecivore mix. Special suplements and additives like High Protein supplement ... and more. Got some problem with food - check on Wombaroo products. Contact me for more information.

Wombaroo milk is more expensive, but then you have to satisfy yourself that the others are up to the job. They may well do it better in many cases.