Regardless if this comes in a Stuff Pack or maybe even the upcoming Family GP, I'd love to have stuff like that in the Sims, for it would increase the joy of playing a lot. There are, imho, not enough interactions with children, be it child and adult or child and child. When my Sim child has a friend over, nearly all they can do is talk. Other than that, the friend will play on the computer... alone. (It seems strange that all guest go to my Sims' computer anyway.)
There need to be more interactions, other than talking. An Ouija board could be fun, as well as sitting on the floor with other Sims and playing truth or dare, card or board games. Playing board games is something I really enjoy, so it's sad that they can only play cards or llama and only on special gaming tables. When sitting together, two ore more children should be able to play with dolls. Without having a big doll house. There should be dolls or doll packages in buy mode, e.g. including doll cars, boats, benches or swimming pools. It could save space in smaller homes, where a doll house just won't fit.
Other things fitting in this 'sleepover theme' would be pillow fights and jumping on beds and couches.
Now coming to CAS and buy mode, I totally agree, that there should be more sleepwear and bedroom furniture. Cute little babydolls, maybe even inspired by Japanese Lolita fashion, would be really nice! More nightgowns. More pjs. Maybe more underwear, for some Sims could like sleeping in their underwear! There aren't many options, so I like to see an increase for those for all ages.
Also more bedroom-furniture-sets, similar to the one in the kid's SP. Single and double beds in modern and expensive, as well as in a romantic style, for example beds with metal frames, maybe decorated with cute (strings of) lights and pillows as well as cute floral patterns. Compatible to that: nightstands, dressers, mirrors, dressers with mirrors on the door, desks, chairs and vanity tables. I'd like for EA to add objects, which they added in one pack also to one or two other packs, to have different styles and more options, but I'm pretty confident that they won't do it for marketing reasons.
Oh, and cutrains! A bunch of compatible bedroom courtains. That be nice^^