Wisconsin has some great single track for mountain biking. Plan a visit and stay in Madison, WI and hit the Southern Kettle Moraine forest state park at the John Muir Trails, Make sure you are advanced with the rolling hills, sharp turns on the 4 mile blue loop.
Took my daughter there this year, and after 5 trips she can tour (not @ race pace) all the loops on a kids mountain bike.
Other system is the Chequamegon Forest in nothern WI that host the Chequamegon Fat Tire Festival
which is the biggest mountain bike festival east of the mississippi river.