PSA: your local fire precinct (in the USA, probably England too) can do a car seat installation and fit check for you. Strap your kids in, and strap them TIGHT. It can be very easy to install a seat improperly, I had mine checked when I was a new parent until I was confident I had it right.
Before anyone flames me for victim blaming I am 100% not blaming the mother. It is quite possible that at those speeds the child would have died regardless, and the driver deserves every day in jail that he spends.