The High Trestle Trail is great for biking with kids. I like having long stretches where we don't need to worry about traffic and it's wide enough that faster bikes (in other words, everyone else biking) can pass them.
There's parking and restrooms available at 1st Street and 18th Street in Ankeny. If you end up at 1st Street, definitely have meal at the TrailSide Tap. The food is outstanding and it's kid-friendly, especially the outdoor patio.