The actual extra risk of transmission surely depends on the ages of the children.
Two five year olds in the same class at school, who are in a classroom together with no PPE or social distancing, have 30 hours a week of close contact in which to spread the virus, and then inevitably take it home to their parents.
Those same two five year olds, with parents, meeting outside at a playground do not in any way increase risk, those two individuals and their families are already swapping germs in circumstances that favour transmission far more than being outside do. The risk is actually lower when meeting out of school!
For older children, who are more socially distanced at school, and more likely to be standing close to each other chatting if they meet socially, rather than running around, the extra risks of transmission from meeting outside of school are greater.
But we have a blanket rule that doesn’t take account of actual risk in individual circumstances, so those in the first scenario are expected to restrict themselves for no benefit.
I guess it’s probably the only practical way to write the rules, but it sucks if you’re in a situation where you’re restricted for no benefit. And it also leads to less compliance across the board when people can’t see the point of the rules. I can understand why people want to bend rules when they don’t make sense.