Isla won't be any more restrained in a high backed booster than she would be in a seatbelt, as it uses the adult seatbelt. The seat itself just places the child a bit higher up so that the seatbelt fits properly (it won't cut into her neck etc). Children's skeletons are still developing whereas adult ones are fully ossified, so can withstand the impact of a crash much better than a child (even if the child is as big as the adult, their skeleton is not as developed). Some adults would do better with a booster anyway, as it can ensure the belt fits them correctly. Being well built unfortunately tells us nothing about how her skeleton has developed insider her body either, which is the key factor in whether and how well a person will survive a car crash. There are tonnes of things to criticise Sarah for, but this isn't one of them. I'm not having a go at you, Betty, as clearly your children are grown so you don't need this advice any more anyway. Other parents of young children may not know all of these things, and sometimes influencers do get things right (accidentally or not!)