This sounds like a perfect time to use a ‘child-of’ constraint. Funny really
Depending on how she is holding the baby, you could add the constraint to the baby’s torso/hip (COG) bone, parenting it temporarily to (say) the mother’s torso, or forearm bone. Then you can freely animate both characters but the baby will be linked to the parent, err. I mean, mother.
So just pose both characters as you want them, then add the constraint to the baby’s bone (in pose mode, if my memory is correct). After you add it, the baby rig will probably jump to a weird position. If so just press ‘Set inverse’ in the constraint panel and it should snap back to where you put it. Then animate away! No counter-animating required.