M.Q.
Hi S. ~ I just switched my daughter from the britax marathon to the britax parkway booster she's 5 & loves it.... she is just a little over 40 lbs and tall. one of the things that I really loved about the britax is the secure guard it looks like a belt in the middle between the legs that works w/the seatbelt (works with the vehicle safety belt to prevent the child from sliding under the lap-belt portion of the safety belt (submarining) during impact, thus minimizing the risk of abdominal )injury. I purchased the booster from chitchatbaby.com w/a coupon it came to 95.99 free s/h no tax.
http://www.britaxusa.com/car-seats/parkway-sg