Jsfein,
I believe the bunk beds are standard size so the top bunk is probably high. We just bought bunk beds for our sons' room. We knew they were to little but wanted to decorate the room and get them furniture.
There is a big warning label that states that kids shouldn't sleep on the top bunk until they are 6.
My 4 year old sleeps on the bottom bunk.
Maybe they could sleep together on the bottom bunk. Or you could request bed rails and let them sleep together on a queen.