Girl was about 3 or 4. DS got off the zip wire and brought the seat thing back so that the girl could get on it but there was no parents in sight and he didn't think she should get on without someone watching here.
The dad turned up and when the girl complained the the 'big boy' wouldn't let her get on, he told DS#1 to mind his own business
AIBU to think that he was right to be cautious. She was so small she could hardly reach the seat.