I know you said bunks were out, BUT I saw these amazing bunks at IKEA today. They are very low. I'm only 5'3" tall and I could easily have reached into the top bunk. They say they fit twin matresses. I love in a tiny house and my 3 year olds are outgrowing their convertible toddler beds. I was thinking of 2 of these. Putting my best behaved girl on the top and 2 on the bottom. Maybe you could hold off for 6 more months, put your 2 best behaved on the tops when they are 4 and move your then 2 year old into the bottom mattress of the other? Or put a pack and play under the top bunk of 1? Truly, I am a worrywart and even I felt my kids would be fine in these.

http://www.ikea.com/us/en/catalog/products/10123996

Anna Lucia, 4lbs 7oz
Matthew Ronald, 4lbs
Sophia Christine, 3lbs 13oz
Born screaming 2/5/07!

http://fruitsaladchronicles.blogspot.com/