I would just tell the parents they need proper foot wear ..... I think you can buy rain boots starting at size 4 or just have them buy a cheap pair of running shoes ... They will get wet and muddy but change their socks as soon as they get inside ..... I told all my parents their kids had to have appropriate outdoor wear cause we go out side .... Two of my parents insist on sending their kids with fabric mittens and the kids dig in the mud and sandbox they dip their mittens in the puddles and everyday I send them home in a plastic bag ... Maybe next year they will get the drift