Would it be possible to use cell phones instead? (Will the kids stay with one of the adults, or are they old enough to be on their own?)
I think cell phones work better than radios. Many other families will probably use radios, too, and you often have to listen to everyone else's chatter, no matter how many channels your radios have.
I think cell phones work great! We had a group of four adults last year, and it was a great way to keep in contact when we split up. Saves you from the inevitable "Wait, there are two entrances to this place? I was waiting at the other one" problems. Plus, if you want to change your plans, it's easier to stay in touch by cell phone, rather than have to go to the appointed place/time and then make your plans.