My experience is as the kids get older (8 or up) they will want to participate. If you can't have an equally fun activity going on the side, you end up with angry kids (because they feel left out) and angry parents (because they feel they can't actual participate and must manage kids all the time).
I recommend for kids ages 8 and up is for there to be kids groups just for kids. That way the kids have there own dedicated activities and time. I suggest not having them at the same time or place.
Something like: http://www.adventuresforyouth.com
Then as a parent do not feel obligated to bring your kids to fighting because you know they have there own fight time. This also means that you need to create play dates or child care when you are at Belegarth. But in the end I think everyone has more fun.
I say this as a part-time father of two wonderful kids (age seven and age six), that we've been considering taking with us to events.
My biggest problem with this absolutely fantastic idea, is that we then have to borrow a huge page from the SCA to facilitate this. Note: I'm not saying that it would be bad to borrow, but the logistical aspect of it is pretty daunting.
The SCA requires full background checks, etc., on those that work with the "children's groups". They maintain a lot of bureaucracy infrastructure to support the program. And (unless my information is wrong on this) they have a highly increased insurance rate for it.
Now, we could totally go that route, and as more of us join the ranks of parenthood, I'm sure we'll see an increased demand for something like this. But at present, the population doesn't really support it. Belegarth is primarily the young adult, single (or at least no-kids) crowd. And the systemic implementation of this... I don't have the energy for it.
Though, finding something like adventuresforyouth pre-available would be awesome.