In April, I am taking my students on a trip to Winter Park, CO. We are flying into Denver and then taking a charter bus the last 60 miles. My son, who will have just turned 4 will be with me. Right now he is about 33 lbs and 38 inches tall. Would you be comfortable with him on the charter bus? There are not seatbelts, so I can't buckle his DC in, although I will be taking it on the plane.