It may depend on how complicated the release is. One possibility is that although Maddie might not be strong enough to press it - I think they are normally designed to stop tiny fingers from being as to release them, Sophie was laughing and sitting next to her. Sophie might not realise it is a dangerous thing to undo her sister's seatbelt. It seems there are 3 explanations - the seatbelt wasn't done up properly/is faulty, Sophie did it or Maddie managed to bypass all the baby proofing protections herself.How on earth does a baby manage to get out of a car seat?