ive taken my youngest in a 2 wheel trailer since she was about 13 months and she loves it, never off road yet, though will do some forestry tracks with her an my holidays.
My older girl was on the tag along (1 wheel bike attatched to my seatpost ) till she was about 5 1/2, now shes on her own bike. We did some pretty rough rides like that both on man made and natural trails, she could manage up to around 12 miles before she got crabbit, but mostly she loved it and never fell off once. They have no fear if they think your in control. You could use her as kind of nitrous to give you a boost up hills if you shouted at her to pedal, though i think most of the time it was me doing the work..