Why do children need to leave the plane during a refueling operation?
You can find interesting lectures about the refueling in a Airbus Flight Operation Briefing Note or in the CAP 748.

When they have children leave the plane, it is probably a company thing! There are no such rules that all passengers have to deboard a plane during refuel (with JetA and similar). But, as you can see in the Airbus FOPN, there are some requirements that need to be met. Each manufacturer as well as each company probably has its on way of interpreting  of the rules and regulations issued by the CAP and the FAA. 

Summing up, it is basically by the pilots and therefore the company's discretion what to do with the passengers (or children) on board during refueling as long as saftey requirements are met!