Another factor is that it's often not made clear when child status ceases, with some supermarkets setting them aside for under-fives and others for under-12s.
It doesn't matter what the supermarkets say, it has to be in legislation for it to be legally enforceable.
If the supermarket want to "hire" the space to you for a set period of time, then the rules for that hire must be clearly set out before you park/enter into a contract with them.
They can't add conditions later, and the conditions they make must be clear, lawful and fair.
What is a "disabled" bay ? Fairly obvious, for Blue Badge holders who must clearly display their badge whilst parked there.
What is a "parent & child" bay ? I have no idea, does this mean I can use it when I'm out with my child, who is in her 20's, and strongly objects to being called a child. Does it mean I can't use it if I take my granddaughter out, as I'm not her parent ?
We can all use common sense, but common sense isn't legally enforceable.
Just as an aside, the ticket can ONLY be for lost revenue, ie if you park for two hours but only pay for one then all they have potentially lost is a payment for one extra hour, anything more is a fine, and they can't fine you, only a court can.
I'm not agreeing that taking up a parent/child bay is right, but neither do I agree that the tickets are either, and two wrongs definitely don't make a right, so just ignore it.