As far as I'm aware, you don't get 'chosen' as such. When we stayed at VWL in April, we asked at check-in if it would be possible and were lucky that there was one opening during our stay. I am not sure about minimum age but they had no problem with our 5-year old son being up there. They are very clear about the safety issues (a few trip hazards up ont he roof) but, ultimately, it is up to you to watch the kids and make sure they're not doing anything dangerous.
I hope you get lucky as it really is a fantastic experience. There are some pictures and a mini report of our experience in my trip report (link in my signature - Fred, Wilma and Family!)