Haldon, as mentioned above. The green trail is doable by a 4yr old on a balance bike/little pedal bike. The top blue trail is not much more difficult, but the longer blue loop down the hill is slightly more technical (not at all technical really, but might be a bit much for a nervous wife or young kids).
You can park for free at Darts Farm, and then cycle along the estuary on a very good cycle path all the way to Exmouth and back. Couple of nice pubs along the way too. We’re doing this tomorrow with the yearling in the trailer, and the 4 1/2 year old on her pedal bike (she’ll probably do the return leg on the trailer).
Worth having a look around Darts Farm, expensive shopping, but a very nice deli, and really excellent fish and chips. The Cotswold Leisure shop sometimes has some bargains too.