From the history described, the incident appears to be very low risk for rabies. The dog only collided with the child while the child was wearing full pants, and you did not notice any bite, scratch, bleeding, or broken skin immediately afterward. Rabies is not transmitted simply by a dog touching intact clothing or intact skin.
Next Steps
Since the dog is known and reportedly healthy, it should be observed for 10 days from the date of the incident. If the dog remains completely healthy through the 10-day observation period, this is strongly reassuring regarding rabies transmission from that incident. Because the child has also previously completed a rabies vaccination course.