I think the Glenbourne pump track in Clayton Park s quite kid friendly and fun. You can pop on it before or after doing the Mainland Linear Trail (big hills, but it’s wide crusher dust) and Belcher’s Marsh Trail.
First Lake Trail in Sackville is a nice crusher dust trail along First Lake. Second Lake Trail across the road is another crusher dust trail that you can extend the ride, but has big hills. There’s a new trail out from the Second Lake Trail parking lot that is relatively flat that goes back along First Lake Drive and comes out across from the ball fields.
A little further from Halifax,
Reservoir Park in Wolfville has some beginner friendly trails and a skills park with many newbie-friendly features.
Kentville Bird Sanctuary has lots of nice doubletrack, and flowy singletrack.