Try the Marin Trail in the Gwydyr Forest, not too technical and lovely views. The climb up from the official start is a bit demoralising for the youngsters so I would recommend driving up past Swallow Falls and take the lane up the side of the Ugly House (very steep and narrow) and park where you meet the trail for the second time, this means you can ride a couple of the loops and not loose too much height, when you come to the crossroads above Llanwrst, leave out the loop down to the carpark and this will keep you up in the forest. Trail map is downloadable from the FC website.