I really like AAA ATV Rental. If you need to rent an ATV for a real trip or just cruising around the Paiute ATV Trail (one of the U.S.A.'s leading off-highway vehicle trails , over 200+ miles of trails), they're a great place to rent whatever you need for the trip. They go ATVing all the time, and really know the trails.
Their ATVs are all electric switching -- no clutch -- so anyone, any age can drive them.
You can drive ATVs on the streets throughout Richfield - helmets required, but that means trailers are not needed. There are designated streets that lead tot he trails, but in actual practise, as long as you're being careful as you drive, and you're not an unlicensed driver on Main Street, then you're okay. The State of Utah has a training program just for kids riding ATVs you can look into if you like.
Their web site has lots of information including a map from their Main Street location out to the trail that leads to Cove Mountain, camping areas and Monroe Mountain. You can see their web site here: http://aaaatvrental.com/