We took our children to Mackinac Island two years ago. Our daughter (then 11) just loved the island. Our son (age 14) enjoyed it too. I think 2 full days is probably about right --we tend to like to go full steam, so maybe 3 days would be more leisurely.
Be sure to rent bikes, see the fort, and if you want to bike to the middle of the island there is a wonderful old golf course that my husband and son enjoyed.
This was our second trip, and both times we stayed at the Liliac Tree. It would be perfect for a family because they are two room (very spacious) suites. It is also located on the Main street, so you could let your 10 year old explore a little bit and still keep a close eye on him.
Emjoy a carriage ride, the Butterfly House would be neat for the younger kids. Be sure to bring sweaters and sweatshirts for evenings. It can be cool.
Hope this helps.