Dogwood Tree Dry Ends
While I cannot be positive with just a picture, my best guess would be leaf scorch. I think these might be our common flowering dogwoods, Cornus florida and not the Kousa dogwood? If so, they would be growing in the wild as an understory tree, partially shaded by taller trees overhead. I am wondering if your trees are in full sun. While dogwoods will grow in full sun, they do often get leaf scorch. Regular watering does help but we have had a lot of hot, sunny days recently. It may not be possible to supply plants with enough water to avoid leaf scorch until they are bigger and have more expansive root systems. You don't want to overwater as that may cause its own set of problems. So I would continue doing what you are for now. Make sure plants are mulched and hopefully as they become more established with a larger root system, they will be able to take up enough water to avoid this problem except during excessively hot, dry summers.