Leaving an autistic child alone may not be the ideal behavior
Discussion
It's impossible to be nonstop hovering over a kid. It's physically impossible. Maybe mum was in the line and girl just went to play few meters out, otherwise she'd throw a tantrum.
It’s a hard situation but the playland is in an entirely different room than ordering. As a parent I know it’s hard to juggle stuff like this. Guessing would’ve been best for the kid to stay with mom until food was purchased and then they can home to the other play room.
Or locked in the toilet stall with her😏 Girl could simply and silently wander off.