What's the worst that can happen? He might fall off and bash his head on .... some soft grass! Still unlikely as he's confident on his bike and rarely falls off.
There are no cars in the park (strange that!) but still I get surprised/shocked comments from other mums.
Yes, when he's older cycling on roads, of course I shall insist he wears a helmet. But do we really need belt and braces in a grassy park?
He's 8 yrs old btw.