If the 8 year old child is skinny enough then yes the 8 year old child should easily be able to fit into Pampers size 7 or even pampers size 8.
I have a son that is 8 and he does wear the parents choice size 7 diapers from Walmart because of bed wetting and they work just fine for him.
He was wearing Goodnites but they were leaking on him so I got him to try and use the Parents Choice Size 7 diapers instead and they fit him perfectly and work great.
So since my 8 year old son fits into the Parents Choice Size 7 diapers your 8 year old should easily fit into Pampers size 7 if he or she will wear them.