we bought for dd a CoPilot Taxi bike seat. It has a bar across her lap and very deep sides. We looked at others (Hamex and some others) but they didn't seem as sturdy or have the deep sides like the CoPilot did. It takes a bit of getting used to cycling with her on the back, especially getting off it, but it's been great. Must use it more
there are a few old threads about this as well that I read before we bought the seat - just type in bike seat.