To be fair, Tesco are definitely in the right in this instance.
It is against the law for an 11 yr old to purchase fuel in the UK and as he was the one with the money, it was him that was making the purchase.
No different really to a young lad in the restaurant of a pub being sent to the bar to purchase a pint for his dad.
This has always been the law, my father fell foul of it when we had a petrol forecourt back in the 1960s.