I've got a feeling that the sentence: "These restrictions do not apply for families with children." is intended to mean "young children", i.e. not adults. Clearly if a husband, wife and 1/2/3 children, all under 17, came on holiday then they would all use the same hire car. This is just my best guess. If the hire car company can't tell you then I don't know who can except, possibly, the police. I say "possibly" since a friend once asked 3 traffic policeman who were stood talking together, what the general speed restriction was on the New National Road, i.e. in the absence of specific speed signs. He got 3 different answers.
PS Is it just me or is it beginning to feel like "The next episode of Soap" for those old enough to remember?