Do you have bunk beds and like the matresses you chose? I'm having a hard time finding a 6" mattress that is actually comfortable but won't break a budget.
All of the mattresses we've looked at are 10-12 inches thick but that won't work for the top bunk. We need the guard rail to be above the mattress so about 6 inches is really the max.
We just looked at the Slumber 1 mattress online so I was glad to see someone mentioned one of them!
Try BJs or if you don't have one near you Costco. They sell the old style of mattress that fits up top and is very reasonably priced. I paid @$100 for mine.
We just have a regular mattress on ours, but it's a very cheap regular mattress and so is only about 6 inches thick. Our kids are 5 and 6, they're not too concerned about mattresses yet and they still have accidents too often for me to warrant getting them a nice one (to me at least).
The one thing I will really recommend is getting a piece of plywood instead of a bunkie board to put the mattress on. They hold up a LOT better, and will keep your mattress in better condition as well. My mother was a furniture sales person growing up and this is one of her tricks