Warning: Your child should only watch you do this and observe since it does involve the use of candles and matches.
If you have a jar type candle with a lid this one is super simple and gives them something to think about. If not use a birthday candle and melt a little of the wax, blow the candle out, place in melted wax on a plate.
Light the candle. Tell them how flames need oxygen. Now either place the lid back on your jar candle or take a glass and cover the candle on the plate with the glass.
Ask your child what happened when you covered it? Show them you took away the candle’s oxygen supply so it could no longer burn.
Super easy experiment that gives them a little bit of chemistry to think about. But for the love DO NOT LET THEM HANDLE THE CANDLES AND MATCHES. (I know most of you have enough common sense to know this, but just in case…..)