L.D.
What fun!! White Rock Lake and the Katy trail both have trails that are off of the roads, but they can be very crowded, which can be intimidating for a new rider (even adults!). The west side of White Rock around the TP Hill area is probably the easiest place at the lake. The hard-core cyclists use the road there, and the trail is a nice surface. You'll run into construction about a mile from TP Hill, but that may be as far as you need to go for a while! Good luck and don't forget helmets - they're required for all ages in Dallas and the police have been writing tickets at both locations!