UK Athletics have clear rules on how far an U9 should be competing in and it's well under 5000m.
Parkrun is a murky world when it comes to what is basically very young kids running and I'm sure when a kid drops dead (unfortunately almost statistical certainty in the next 5 years) all hell is going to break loose.
They are trying to have a caveat
"(UK Athletics) say children under the age of 11 should not take part in open competition of 5,000m distance. However, provided these children are not put under any undue pressure to perform by teachers, coaches, parents or guardians they may take part for fun. Children aged 10 and younger must be accompanied throughout the run by a parent or guardian.”
However you can't disclaim away your responsibilities; and I'm sure you run alongside your child