Wouldn't this defeat the purpose of playing a mother raising a child
I don't know that there's one "the purpose" for playing a mother raising a child. You'd probably need to ask the OP what the purpose is in this case.
But, that frames the following question well - is the player's purpose in this compatible with the purposes of others playing at the table?
I don't know how much of the purpose is to actually play raising a child - that is a very long, very slow set of processes, for which we have few mechanics in the game. Raising a kid takes years, but our rules are for rounds

I would suspect, "play a character with a major obligation that constrains their action," or "play a character with an important emotional bond unrelated to adventuring" are likely purposes. These are not necessarily at odds with having the kid removed from the action, or transformed after a little while. After "protect the baby" gets old, that baby isn't really helping the story develop. The issue becomes a constant, not evolving much on the timescale of adventures.
See, for example, romances in most fantasy stories. Stable, healthy relationships are generally avoided in long-term stories, mostly because they aren't dynamic story elements. Meaning, basically, they are boring.
