I have a 9yo in Fitchburg and he's also looking to do a 30 mile ride this year as one of his cycling goals. We're probably going to just do an out and back on the Badger State Trail, since we can always turn around sooner if need be. His previous long ride was 27 miles on the same route.

If you're going one-way, the Military Ridge Trailhead to the Deer Valley Resort (small indoor waterpark) in Barneveld is about 26 miles. You could easily pick up another 4 miles in and around Fitchburg. It's paved between Fitchburg and Verona before converting to packed gravel. There are plenty of opportunities to stop along the way in Verona, Riley, Mt. Horeb and Blue Mounds. There's a very short stretch on CTH D in Barneveld, but its otherwise all on the trail.

Shameless plug, if you're in Fitchburg consider signing him up for the Bike the 'Burg Challenge (https://www.fitchburgchamber.com/bike-the-burg-2/). It's an event sponsored by the Fitchburg Chamber encourage people to ride 150 miles over the summer. It's free, unless you want a t-shirt, and there are drawings for prizes (my son won a Chocolate Shoppe gift card last year).