I've taken my boys (6 and 8 now) on a few rides in Circle C park trails between Escarpment and Mopac. The north bank trails are really good for that age, just a few rock gardens without much up/down climbing. Post-SATN ride good options are Waterloo Icehouse on Escarpment just north of Slaughter (has beer, obviously and an outdoor playground) with an ice cream shop right next door to boot. You'll have to cross Slaughter but easily doable at the light at Escarpment. My boys also like Zpizza after a ride (few doors down from ATX bikes). As The Tip said, you can also hit the gravel trails west of Escarpment to get to the playground/ball fields. The single track on that side is probably a bit above level for now as there's more up/down and tech on that side of Escarpment.