Welcome to the Great American Roadtrip Forum.
What lucky boys to have grandparents like you.
One way to keep them interested - and I know others will come up with more local attractions - is to get the boys to write a journal of the journey. A great thing to look back on in years to come. Works best, if you too are writing a journal.
Each night before bed they can record the most memorable things of that day. This can be done in text, drawings, stubs from entry tickets and maybe something obtained or found along the way. Later you can print photos to add and illustrate the journals.
Lifey