Advice about in-laws, please....?

BIL HAS A BAD HABBIT OF LETTING KIDS WANDER AROUND OUTSIDE UNSUPERVISED (KIDS ARE AGES 4 AND 6). THEY ALL LIVE WITH MY MIL/FIL. WHEN WE WE GO SWIMMING, THE KIDS APPEAR FROM NO WHERE AND WE ARE EXPECTED TO WATCH THEM. MY KIDS HAD FRIENDS OVER TODAY AND ONE OF THE COUSINS CAME OVER TO USE THE POOL. FIL WANDERED OFF. DON'T KNOW HOW TO HANDLE THIS SITUATION. I WALKED OVER COUSIN BACK HOME AFTER ABOUT 30 MINUTES IN THE POOL. I KNOW THERE IS GOING TO BE MAJOR DRAMA ABOUT THIS. NOT SURE WHAT TO SAY.