I stayed at the Homewood Suites hotel in Liberty which is about 5 minutes drive from the airport. and they have a shuttle.
It is difficult to walk with a 4 year old in any case and only you understand how your child may react to noise, a long line, pushing and tons of people.
Maybe going to the hotel with a shuttle, leaving the family to explore the hotel and settle into some kind of routine and then you going to pick up the car is a much easier option for you, your wife but especially your child.