I am flying with both kids in June. We have a 30 min layover in Denver (yikes!) so I am trying to minimize carry ons. I do not need to take seats, since I can send them ahead of time with my parents, who are driving. But I know dd will need a seat on the plane, so taking either the Radian or the TFP for her. For ds, I would love to not take a seat, but will he fit in the airplane belt? He will just barely be 6yrs, and his weight is around 37/38 lbs. He is 43 inches tall, so skinnyish.