I would say for now, while the weather is still hot, leave them with the alternate waterer so you know they're getting plenty of water. Once things cool down you can remove the alternate.
I taught my girls to use their nipple waterer by having their original waterer in the same area, so when they went looking for water I'd tap the nipple and let a little water trickle down. They quickly learned to look up instead of down for water and within 10 minutes all of them had learned to use it.
When it's hot out I put out extra water for them (in a planter saucer) so they can stand in it and cool off their feet and easily get a little extra water to drink too.