I've never done Cambridge so can't comment on that.
I always go to Whitby. One thing to be aware of is that since the government changed licensing laws pubs do not allow children (and that means age 17 and under) on pub premises in the evening. Before that, well behaved ones as part of folk families were able to go into rooms separate from bar areas. In many pubs the cut-off time is eight, which is often before people get cracking on their evening's sing or play (they can stay in the Endeavour until nine). That one factor made a big difference to what we were able to do as a family.
Official festival concerts are not affected, and there has been a big singaround at the Friendship Club where kids were not banned.
I would also echo Noreen's comments about Warwick Festival. There is something on site for children of all ages, and the option of going into town (a shuttle bus gets you there, but it is a fairly short walk) for some on street entertainment and a decent Saturday market.
Quack!
GtD.