The Tallgrass Trail is known for ticks and wild parsnip, so you definitely want to stay on trail. This is a great time of year to visit when neither are a concern.
I highly recommend visiting when the Visitor Center is open, which is Thursday through Saturday from 9 am to 4 pm. It has hands-on exhibits about the tallgrass prairie ecosystem, a scavenger hunt for a wide range of ages, free equipment to borrow including binoculars, snowshoes and children's activity backpacks. There's also a lunchroom with both indoor and outdoor tables available.
The Prairie Point Nature Store is also located inside the Visitor Center and it has an excellent selection of adult and children's books, but be aware it also has a wide variety of very cute stuffed animals that your child will likely want to purchase.