My kids will be 14, 12, and 8. If I was presented with your choices, I'd go to Guatemala. It's intimidating, but it would be so rewarding.
If you can't get your heart settled on going east, then don't go. There are so many other options available to you that you can choose from. Too many long driving trips in a row can be exhausting.
I agree that your boys might be a bit young to backpack across Europe, postpone that until they are older.
If you stay in the states, I vote for Michigan. It's not as far of a drive for you and would still be amazing. You could spend your entire summer here and not be bored. You can visit the family for a bit, then move on and do your own vacation.
I would specifically aim for visiting the UP and visiting Isle Royale NP. After reading your trip reports, I think this would interest you. Isle Royale NP is on my bucket list before the kids grow up. Your boys are the perfect age to go. We haven't gone yet because my 8 year old is still too young to hike far. It's remote and not full of people and rumored to be fabulous. If you decide to go here or anywhere in the UP, do NOT go in June or early July....the bugs will eat you alive. August is your best bet. Spend June and early July in the lower peninsula and work your way north as the summer progresses.
If you like fewer people, go farther north. The UP has the least amount of people anywhere in the state. In the UP you can tour copper mines, visit lighthouses, see waterfalls, take shipwreck tours. Your boys are good ages to kayak around Pictured Rocks. Swimming in Lake Superior is pretty awesome as well. Porcupine Mountains has sites directly on Lake Superior; amazing.
If you're in the lower peninsula I would suggest you avoid Traverse City, Holland, and Grand Haven. You will get the exact opposite advice from many people, but if you dislike crowds, that's what you'll find there. I was not impressed with any of those areas myself. Way too busy.
We have some pretty amazing state parks with sites on the Great Lakes. To get into these parks in the lower peninsula, you must plan ahead. On summer weekends the parks will be full months in advance. If you want more specific info just send me a pm.
Happy planning!
