Actually I've been wondering about this for a while now, I know other places also have that restriction, the basement at "arenan" for example and, if I am not wrong, I think the same is true for the large scene at "arenan" as well, or at least when specific bands play.
First I thought it might be that the sound levels could harm children in other way than just their hearing but that sounds very weird.
I dare guess though that if you actually contact debaser personally they might change their minds, especially if you are ready to pay tickets for the little ones as well, they want to earn money after all as long as noone is hurt...!