Dd 1 2 years rf when she outgrew it (university as only designed to be until 15 mos but she was small, no seats eg any longer then) dd2 about the same as she too was petite.
Dd1 was involved in a motorway high speed crash in her ff harnessed seat and was fine - police said that the seat was fitted correctly saved her life. Make sure whatever style of seat you have it fits your car, fits your child and is installed correctly, this is far more important than rear facing a child who doesn't fit properly at 5 years. Also never have thick coats on, use a blanket over it in winter.