I think that any trailer with a triple bunk layout will all have just about the same bunk height.
If most travel trailers have approximately 7 ft from floor to ceiling, this will equal out to about < 24 inches of head room in each bunk depending on the mattress thickness.
We have a 2306 that has 3 bunks and my DD (10 yrs) is just fine in them.
(However I will add that the bottom bunk is the least popular because it is harder to get in and out of, so, maybe your children will have to draw straws to see who gets that one.)
Good luck with trailer shopping!!
2010 Dodge Ram 1500
2012 Rockwood 2306
DH (Alan), DD (Madison) and Zoey the lab,