History is never easy to explain to children, but today (and all month long) is one of those days it is worth a try.
Just as children grow to the point of adulthood and eventually go out on their own, so can a people grow in such a way that they need to separate from another country and start a country of their own.
On July 4, 1776 our forefathers declared themselves free and separate from Great Britain. While freedom is easy to take for granted, today (and all month long) is the exact right time to talk to your class about it and the amazing country that we continue to build because of it.
Celebrate well and enjoy!