I am from Michigan and my family is still there, so we go to Michigan a lot for mini vacations. My daughter is only 2 and loves waterparks. Recently we went to Frankenmuth and stayed at the new Zehnder's hotel/waterpark. We had a great time, and I love Frankenmuth so we went to Bronner's and walked down main street. By Thanksgiving they will most likely be totally decked out for Christmas. They have horse carriage rides, a petting zoo, etc.
If you head further north in Michigan, closer to Traverse City and such, there are much bigger waterparks though that are much more popular, including a Great Wolf Lodge. Traverse City may have more things that your 8 and 11 year old would be interested in. Also, Boyne is a ski resort and also has a large waterpark, Avalanche Bay. I have not been there but I have heard great things about it. I am not a skiier but I think that by Thanksgiving the ski resort will be open as well, if your kids are interested in that.
Good Luck,
D.