A few comments from SimGuruMorgan in the Quality of Life Q&A on Discord.
Infant caregiving is my favorite part of this update. Base layers is a close second, because there are so many tummy showing shirts I can now wear on my 'morgan' sim. I'll show off some in the livestream! But infants just feel so much nicer now when you play with them and I'm a big family player.
You can put any Top or Full Body Outfit over a Base Layer Asset (which works like tights. To be clear, YOU CANNOT layer two top assets. It's a Base Layer outfit + top or full body.
It's very cool! I have some good examples I'll be showing off in the livestream towards the beginning!
If you count every variant for every asset across Toddler, Child, and Teen+ it is 350. But I think the real magic is how those assets can all be combined with the different assets from different packs.
For this update, all Base Layer assets are in base game. We could add them (pack CAS) for future releases if desired, but that isn't part of this update.