I think we can reduce the risk of fire by only using candles that are more than one inch thick/wide. Also, I have several glass ash trays ( or even glass bowls ) to be used under candle in an emergency. If they faal to one side, they will fall and melt within the bowl .

Also, you can always put a metal tray under any normal candle holder. If a candle falls, it falls onb the tray.

EDIT:
Most important thing though, is where you locate your candles. Always position them in a corner a bit far from traffic, and high enough if you have younger kids.