According to the crowd at the reddit thread "The UR Rider UrRider front mounted child seat" the UrRider Child Bike Seat is the Best value older child bike seat.

The UrRider front mounted child seat solves the problem of what to use when you want your older kid to ride with you on your bike but they do not fit into the rear child seats made for younger kids. The UrRider positions your child in front of you with a sturdy mounted seat and unique handlebar setup that keeps them close. It is a good value at under $100 and can be easily moved from one parents bike to another in under a few minutes.