ItsYaBoi4455 (and probably others have already mentioned it) but I'm making my own post about the limited baby slots because idk, I have my own special baby boy take on it. I have now played the game a good bit and looped a few times and I think that four slots is pretty reasonable for how the game is now. Should it expand it will absolutely need more baby slots but as of right now I think that at most one more baby slot should be available and should be balanced in a more specific way than just money.
I think the second addition baby slot should take up your active item slot.
IE: once you buy the backpack the first time you can buy like, Pandora's box or something like that which will then allow you to - before starting a fight - select it as your active item. When you do that it you can select an additional baby to take into the fight. I just realized it could maybe be an active with a super long charge which will just place that baby for free, but I'm worried that might be either over powered or under powered. either that, or just you can buy that baby with hearts like normal.
I do think baby slot upgrades should be guaranteed (if they aren't already) with maybe backpack after floor one and "pandoras box" is guaranteed after floor two (or maybe after three so its an upgrade that's only available for looping), either way I do agree with people asking for more baby space but I would absolutely want it to be something you have to think about instead of just being a required upgrade. The game is already really neat and absolutely has a place as a hard rogue like take on the PVZ formula so I'm excited for more!