Remember children will be sleeping, studying and probably roughhousing on the beds from time to time, so the entire structure will need to be very sturdy.
The best way to ensure that your child stays safe is to teach him good bunk bed etiquette, which means no roughhousing on the bed or jumping from the top bunk.
If there will be kids and pets roughhousing on it, a durable fabric is ideal.
If you're dealing with a tween or younger teenager, discuss safe loft bed practices with him, like no jumping or roughhousing, before you turn over the "keys" to an elevated bed.
These shoes will withstand roughhousing, but they also keep the feet cozy during quieter times.