Best wishes for you and your family on a safe trip! If you are still planning on coming to Des Moines, I recommend either Ledges State Park or Jester State Park. Both are close to I-80 / I-35 interchange and will allow you to make trips to nearby sights.
In Indianapolis, I highly recommend looking at Turkey Run State Park in Bloomington, IN. We have stayed their twice and it is beautiful.
When you are in Detroit, I would pick the Henry Ford and Greenfield Village over Ann Arbor. Both of those are in Dearborn (west side of Detroit). There is a state park a few miles west called Marbury that is very nice. You can also try Sterling State Park (30 minutes south, but on Lake Erie), or possibly even Camp Dearborn (Actually in Milford).
I grew up in Dearborn, and now live in Iowa. Feel free to email me if you want any more specific advice. Good luck!
2014 Forest River Shamrock 21DK
2012 Ford Expedition Limited
Load Leveler / Sway Bar (800#)
Tekonsha P3 Prodigy Brake Controller
2016 - Four trips, 11 states, 19 nights
2015 - Four trips, 12 states, 17 nights