As many others have pointed out on here, the amount of slots that are given to the player are on the low side. Therefore, I think that there should be some other ways to increase the amount of slots that you have, other than schoolbag. One idea is Birthright, which, on pickup, will randomly select one of your babies and permanently add it to your roster, for the cost of 33 coins. Maybe you get lucky and get lil' chad. Maybe you end up with Farting Baby. Regardless, it adds an extra slot in a new way. Just in general more ways to get slots is (in my opinion) probably a good idea.