I think there are quite some players who only play (young) adult Sims, so I can see why the other age groups are somewhat neglected. I wish it we're different, though, as I always play families. I got the kids clothing kit to show that it was the direction I desired. I'd definately buy an infant or toddler kit, but after all the complaints about the infant patch and that some of the infant stuff was reserved to Growing Together, they are probably afraid of the backlash (like "This should have been in the patch / in Growing Together and the Team deliberately held it back for the kit. What a cash grab.")
It's not easy being EA.