There’s some brutal hills, up and down. 20km of the wrong sort of tracks would break some grown men.
Depending on the kids; parking at the top car park due west of Dunkery by Exford Common and riding up to the beacon, maybe down the other side (rocky but rideable)and retrace or north down a steep but relatively smooth (as Exmoor goes) b/way and then left along the road back up rto the start might be doable.
Or for flatter (again as Exmoor goes) but scenic – park in Horner and then follow Horner water all the way to the Pool bridge camping site, and then retrace. Although it’s all stream edge with babbling water, etc, it’s an interesting path with ca 140m climb along it’s length (and 140m drop onn the way back) – and an ice cream or jammy scone at the end.