Hello. I've taken both my boys since they were 4. The Kidz Field is fantastic, there's loads there for littl'uns to do (although you'll need to keep a close eye on him as it gets very busy with kids going berserk and easy to lose sight - there's loads of stewards in the field, though, and they won't let kids wander off on their own into the festival). Green Kids is great, too, more chilled out/less manic, although he won't be allowed to climb up onto the top of the Pirate Boat.
Â
Best advice I can give is get a buggy with big bike wheels on it, much easier imo than hauling a trolley around for them to crash out into. I've used one of these for years: https://www.aosom.co.uk/item/homcom-18m-2-seat-child-bike-trailer-kid-stroller-w-steel-frame-door-seat-belt-blue~440-008BU.html You can get ones that have a front wheel attachment to turn it into a stroller.