Parents need to stop "asking" and start "telling". There is a red weather warning so the school needs to cancel or postpone the sports day. Ask if they'd go ahead in snow or high winds, and when they say no, ask why this red weather warning is any different. The Met Office doesn't give them out like confetti. I'd simply say my child would not be attending.
And to answer the constant questions about hot countries, this says it better than I can:
Aiden McGivern, a meteorologist, said that people should take the heat seriously. “You might be thinking, ‘I can cope with these kind of temperatures. People go on holiday to Spain and cope with these kinds of temperatures. Why can’t we?’ The fact is in the UK we’re not used to these kinds of temperatures. They rarely if ever happen. We know from previous heatwaves the kinds of impacts they can cause to people’s health . . . even to fit and healthy people.”