Toddler Basement?

I don't think she built underneath the platform, more like she raised the basement platform height.

I saw a video (not the one you mentioned, but I don't remember which) where they made a child-only section of a room by placing a platform and raising the level of it till adult sims couldn't enter that area. They used a ladder or stairs to access the raised section. With a basement it should be even simpler, since they would just be going down the stairs into, as @crocobaura said, a basement that had had a platform put in and raised to the correct height to prevent taller sims from entering. I guess that's how it would work, or you could just do a regular basement and give the toddlers or children a room of their own with the platform height raised to the right level. As the kids grow older, you would simply alter the platform height accordingly.

I love Steph0Sims. It always amazes me that she is s GameChanger since she has no real censor on what she says :D