The best walk for children I can think of is 'the Stevenson Way' starting at Le Puy en Velay in France.
Download (its free) 'Travels with a Donkey in the Cevenne' by Robert Louis Stevenson and have a read. Look up the route on your PC and then investigate donkey hire in the area. Hire a donkey from the Le Puy area, or more than one, take your childen, follow the route and finish at St Jean du Gard or earlier, for example at Florac. The donkey(s) are picked up from there by the hiring company and they return them back to the Le Puy area.
I've walked the route, the small hotels that I saw have paddocks to cater for the donkeys as well.
En route I met 3 German families with 4 donkeys, wives and their childen, all travelling together, reminded me of the TV series 'Wagon Train.' The children loved it!