Default infant outfits will replace the diaper for both the girls and boys. Nothing can be done with that

You can't buy infant clothing ingame, as there was no such option with the game. Either defaults, skintones or diapers...
(I like defaults best, since these are changeable. Combined with my own modified version of Simotized's skintone changers, and I've got everything I need

. I've even figured out how to combine two styles and several recolors, say if I want two infant girls in different dresses and two infant boys in different outfits. I really think I've got to make my own skintone changers for upload soon...)
Infant skintones will show up on both girls and boys, since there's only one skintone texture available for infants. Skintones are also impossible to change, unless you save before the birth, have the baby, go to neighborhood without saving if you're not happy with the outcome, go into the lot again, have the baby - and repeat everything again if you're not satisfied now either... Can take forever...
Only other possibility, which gives the infants several clothing options ingame, is the skintone changers Simotized made (4 of them, in the community mods/hacks area).
(Oh, and just so you know - I've got a huge bunch of different default replacement infant outfits, both girl/boy/unisex, in the two links in my signature (my site - registration req, and Bloom's guest recolor section). You should manage to find at least something you like there

)