Couple of problems. First is when you water a lot when it's hot, you are asking for fungus. However, you have to water frequently for the seeds to sprout. Seeds need water, oxygen, and warmth to sprout. I'd be surprised if they don't sprout due to the heat. If the ground dries out, that may do it, though.

I would keep waiting to see what happens. If you lawn was relatively thick already you may not notice the new seedlings anyways. It also depends on the type of grass. If they planted KBG, you are just now getting to the point where the seeds typically start to sprout. If it's Fescue, that was about a week ago. Even then, all seeds don't sprout at once. It takes a while for all of the seeds that will sprout to come up.

Don't worry about it for now. Keep watering and watch for fungus.