You are the best natural incubator for them. When mom is playing try to hold them (wrap them in fleece like a burrito
), in front of her; if the babies cry and momma comes to get them, hand them to her (if she comes almost immediately after they start crying, this is a good sign that she is not intending to reject them; on the other hand, if they cry and momma just ignores them, it could be taken as a sign of her possibly rejecting them).
The reason you may not want to use the bottle of hot water is because, 1 they could overheat, and 2 joeys can become dehydrated very easily. To check for dehydration do what we call a tent test:
Pinch (softly) the skin between their shoulders, if it goes back down immediately, they are well hydrated; if it stays up (tented), they are dehydrated, if that is the case, you could offer they joey some unflavored pedialyte with a syringe, or similar (just wet its lips, it should try to lick it; do not feed it directly into their mouths, they could drown).
Hope this is helpful.