My daughter is 2 and a half, and we are getting her a low loft with stairsteps instead of a ladder....now granted she doesn't have someone in there with her to egg her on, to fight with or to play with ....but i think she will be fine.
Maybe get bunks that come apart and put them both on the floor as singles, until the boys get old enough to handle them stacked??
Maybe consider a lower style of bunks....like these:
http://www.sendmetomyroom.com/22cabedfor2w.html
http://thebeanbagstore.com/bhhoosierlshapelowloft.htm
( my daughter's is like this , but a different finish and brand - there is no bottom bunk under hers, there is a roll out desk)
Or if you are worried about the ladder - consider something with stairs like these:
http://www.simplybunkbeds.com/twin-over-twin/wood-bunk-be...
Just make definite rules and consequences, they are old enough to understand and obey, I think, but you know your own kids well enough to know if they will.
Avoid this moeel if you don't want to encourage playing...:
http://thebeanbagstore.com/bhalmostbb.htm LOL!