Plenty of nice wide and flat options on the quay in Exeter.
Depending on how strong they are you can safely get to double locks without going near a road, have a pint and a play in their park before returning the way you came. Finishing with iced cream on the piazza.
As they get stronger and more capable there are plenty of options to extend.
Alternatively for a bit of an adventure park at Exton, ride to Exmouth (<4 miles) along the cracking sustrans route and get the train back, however there are some quiet road sections on this route and a few hills that will require walking. Did this with my four year old the other day and she loved it.
Always interesting things on both routes to see.