My kids turn 13 and 11 this year, both have been nearly every year since they were babies. In fact my daughter was 3 months old when we 1st took her. For late night gigs we just used to park the two prams at the back of the Garden Stage as if you were heading to the toilets, hedge worked as a sound barrier.
Anyhow, I digress. Both mine love going, they've moved past the workshop activity area, preferring instead to spend time at table tennis, skittles area. I find with kids they don't actually need a lot of arranged activity, they can make friends easily enough and make their own entertainment. They know the site by the back of their hands (my eldest has habit of sneaking backstage to use the posh toilets😀). Although bad things can and do happen, I feel comfortable leaving them to their own devices for periods of time.
Mornings I tend to have a kick around with a football my eldest brings on the field of the Woods stage whilst the headliners are sound checking, as there is literally no one there.