I have seen this too. Been behind people at the checkout a few times when the parent has been refused due to having their children with them.
The checkout person doesn't know they are related. The people involved could just be acting. If both produce ID then you can tell that they are related although it would be irrelevant as it would show whether they are old enough to buy it themselves or not.
But i am talking about children, not teenagers who want their mum to pick them up some cider to sit in the park with.
These instances have been where they have their huge weekly shop on the checkout along with wine, beer and spirits so i think it is a bit OTT to refuse service knowing full well that the children won't be able to prove their age is over 18 and that it is hugely unlikely that these drinks will be for them.