Sit where the 6 year old can see you (and so can random adults who come across the 6 year old) or
Sit where you can see the 6 year old but don't worry if nobody can see that the 6 year old has an adult visible?
Said child came up to me and my two DCs (older one is 10 and is sensible enough to play alone/with DC2 but I was there). Asked to go on the big bowl swing with them.
I asked where their mum or dad were. Was told they were inside the pub (next to park, you can see park from pub but can't see inside - there are seats outside but they are empty).
I told 6 year old he needed to go back because we couldn't see that there were adults with them.
Child told me "BUT I'M 6 I'M BIG" 🤣
I told child to come with me to pub. Child ran ahead, I checked there was a parent in the window seat, all good, felt I'd done my duty. Didn't talk to parents and child came out again but I knew child was being watched.
Next thing dad came out and shouted at me. I pointed out I could actually have called police and in fact just checked there WAS a parent.
YABU: this is adequate supervision and I could have believed 6 year old
YANBU: parents should be where child (and random other adults) can see they are watching.