Loft beds or upper bunk beds are safe. I think, of the normal base game beds, the emoti-cot (the one with the drawers) should be safe, or at least I never had a monster underneath that one. I guess it's too low. The one that looks like a boat (from the store I guess) is also fine.
Normally, it isn't such a big deal though (not nearly as annoying as the Sims4 version). For the most part, I just cancel the action if I notice it. If I miss it, the scared kid gets an upper bunk that night. At least in my experience, it's always just one of the kids that develops that specific habit to look under beds.
