Why don't you feel like she's safe?
I'm not a big fan of the seats with no back, anyway, for comfort reasons. My dd is almost 7 and about 50 lbs, and she sits in the seat with the back on it, then the seatbelt harness crosses over her. My ped said 8year/80lbs and then she won't need it anymore. My 5yo ds also sits in this type of seat.