Most beach huts have three adult life jackets and a shrug for anybody smaller. We carry child sizes from about fifteen kilos, small masks, and wakeboard bindings that fit a child's foot.
Say the ages and rough sizes when you book and the right kit is out and ready when you arrive rather than being dug out of a box.
Everybody on the water wears a jacket, including adults who swim well. It is not a discussion we have.