
I think some of the kids sleep on futons but there are beds based on the few panels with the kids sleeping hahah. I think they do have the space for all kids to have their own rooms since Shizu has her own room and Kazuki has an office. I think it's cultural to have small kids share the room or potentially sleep with the parents until a certain age.
PS do not quote me on this as its speculation and I haven't done the research to back it up haha

It's a cultural thing. Asian families with little children tend to sleep together in one room, sometimes on the floor, and they prefer small homes as opposed to large ones, regardless of status. There's also the fact that if you live closer to (or in) a major city, houses tend to be smaller (even if they are pricier), so I'm pretty sure that's what's going on here.

Basing from naoto's character, it's probably his decision to live in an apartment. He might feel intimidated or pressured living in a home like hazuki's because he might be thinking that he would be expected to live and act like rich people. We know how afraid/ shy naoto was in taking hazuki's last name so he might feel the same with their life style. Even with his occupation, naoto chose to stay with his current job instead of working with hazuki. So he probably wants to continue living the way he currently does sufficiently enough to provide a secure and enough space for their family to live and grow together. And also knowing hazuki, he definitely does not care where they live as long as it's not harmful and it's enough for the family, and most epecially, as long as he lives with naoto.
Since it wasn't explained how they decided to choose a place to live in, this is just my understanding of the situation lol.
if the alpha is so rich and amazing why are they living in such a small apartment? omega boy u better empty those pockets theyre not full for no reason