We used to ride Dalby when the kids were just starting out. The complete blue loop from the Visitor Centre was a good starting point for them. If they are brave and you are patient, the same loop joins the red at Riggs and Dales, and this was good point to take them to sample the reds, but it is pretty tough for beginners in places, they struggled early on with some of the climbs and steps, and slippy rooty bits may catch out the unwary.
Otherwise, start at Dixons Hollow and mix red and greens. Again, the red round the northernmost loop can be a bit full-on for inexperienced youngsters (but can be shortcutted or chickened on fire roads) but most of the stuff right at the top of the loop back down to the green due east of Dixons is pretty tame.
It’s muddy in places, and puddles galore this time of year, but not impassable. As above, take towels and dry change of clothes