Is there a particular reason you chose Flint? It's arguably the worst part of Michigan, much of it is very dangerous. It was the murder capital of the United States many times in recent years. The police are extremely understaffed. They made a documentary about it a couple of years ago.
Flint Town | Official Trailer [HD] | Netflix
If you must choose Flint, have them stay in maybe Birch Run or Grand Blanc (nearby, especially Grand Blanc, but Much safer).
You could fly them into Detroit Metro (which is actually in Romulus) or Lansing and have them stay near the airport as an alternative.
If you're just flying there all together, renting a car at the airport, and driving off, you should be absolutely fine.
I just checked and Lansing to Hulbert is almost the exact same distance as Flint and a LOT safer. Romulus (Detroit Metro Airport) is about an hour more.
Driving may be a good alternative since you could bring a gun/guns. There's lots of outdoor stuff to do in the U.P. I'm not the type to feel distressed about not being armed 24/7 (even though I generally am), but Flint and Detroit (on your route) are two of the most dangerous places in the country. 99.9% you're fine, but if something goes sideways...
Also you may not be that well prepped for snow, especially Upper Peninsula snow, being from a warmer state. You'll want coats, boots, hats, gloves, heavy socks, etc for everyone. Slight but real chance you could have your trip interrupted by snow or be snowed in for a bit. Not usually dangerous but it helps if you have stuff. Again might be better in the car. The Upper Peninsula is some of the northernmost territory in the U.S. and it gets a lot colder than the rest of Michigan, which is why it's such a Winter recreation destination. It's pretty much a different state, mostly in a good way.
If it snows/freezing rain/sleet whatever during your drive, SLOW DOWN. Especially if you aren't used to driving in the snow. We kind of joke about Southerners not being able to drive in the Winter but there's nothing funny about sliding/spinning off the road with your family. I live in the Detroit area, drive to Lansing three days for work (about 80 miles one way) and on slippery days there are always cars slid off the road. The ditches can be deep and it's no laughing matter to be stuck in one, especially if nobody sees you right away. Slow down/take a break if you need to if the driving gets bad.
I live near Detroit. If you find yourself in that area and want to take a break/get some dinner or something, I'd be happy to show you around. With a little notice we could make an Arfcom thing out of it.
Also, you probably know this already, but it's not exactly a thriving metropolis up there. This is a plus in many people's book, but plan accordingly. If I were going up there I'd have at least loose plans on where I was going to shop for necessities/food/things I forgot/sporting gear/whatever, where I was going to eat, how to find medical care/EMS (I imagine just 911 but the UP can be very rural, police/ambulance can be an hour or more away. Again, can be a plus in many cases but plan ahead). If you like to eat out, there are a few restaurants, etc. If your kids get bored easily, plan for that.
Good luck, and have fun!