I think it should be fine, you can put up a rail or net type thing (not sure what they are called) but that will keep them from falling off. We are thinking of doing that with our boys.
no when my DD turned 4 we got her bunks.
DS is still co sleeping with me so when he is 2 he can go in the bottom.
she is 5 now but a whole year in the top bunk she loves it.
((no the room isnt messy like that normally they were going nuts thowing clothes toys everything everywhere! lots of fun :) ))