* We drove on a big highway called an interstate to visit Grandma!
* The interstate has lots of cars and trucks going really fast.
* Interstates help us get to different places all over the country.
* You can see signs on the interstate telling you where to go.
You could also try:
* The interstate is like a giant road for cars.
* When we drive on the interstate, we can see lots of trees and mountains.
The best sentence will depend on the context and the child's age and understanding.