Never. I can only say they are the most dangerous thing you can give your child.
Please don't.
My best friend in Jr. High and High School lost her young son in a bunk bed fall. He was in elementary school and was not jumping or roughhousing. He simply fell out of the top bunk and died.
It was a simple accident that most likely would not happen to your kids but why take that chance. Your kids are much younger than he was and they are much smaller too. Please consider other options.
There are trundle bed that fit under a day bed. They pull out at bedtime and lift up making the day bed a bigger bed. There is also the kid of bedroom set that has a corner desk and one bed slides into the corner unit during the day. Only a portion of it stays out. This type of bed set fits in the corner of the room and is especially good for smaller rooms.
Here is a more modern set, the one bed doesn't slide inside the corner. They are not too horribly expensive if you shop around. You might even be able to make one yourself out of existing furniture too.
http://www.potterybarnkids.com/products/belden-corner-uni...