Eh, do you have the ape already? If I recall, the child becomes an ape if you have any other type of creature. The trick is to save the game before you take the child to the house (may be to late), change for the ape, load the game (creature files save automatically and are retroactive, so you'll have the ape), give the witch another child, ? ? ? ? ?, PROFIT!
I have an ape (now a chimp) this time around, so I can't test it, but I recall doing this to my mandrill once.