Relax your standards. Crooked rows or weeds as pets are fine. Transplanting is fun, even if your child plays with plants the way they move action figures or Barbies about. But remind them that plants' roots need some time to grow in one place. Leave room for good old-fashioned digging.
Holes are a highly popular landscape feature.
Look for worms. Add water, and frogs appear. Model the message that some insects are beneficial, and even destructive bugs are highly interesting. Don't expect kids to do all the watering and pest patrol.