They're growing fast, they need water every day. Why do you think you need to water less often?
If it's worry over the fungus thing, really think about plants that grow outdoors. All sorts of fungal entities live in regular garden soil. They're part of the natural world and plants deal with them just fine. They won't hurt you either, you breathe them every day - where do you think the spores for the white fungus came from? The air you're breathing in your home. Not harmful.
Water those little plants every morning. If they're droopy in the afternoon you might need to water them again. Those cups are going to be mighty small for those plants by the time your science fair happens.
