A 6 year old boy or 6 year old girl with a small enough waist size should be able to easily fit in a size 6 Luvs diaper or even pampers size 6 diaper.
If the 6 year old does not fit properly or good enough in the Luvs size 6 diapers or other size 6 diapers then you might try a size 7 Pampers Cruisers, Size 7 Huggies Little Movers or even Size 7 Gentle Steps diaper.
Dollar General sells their own brand of diapers in sizes up to size 7 diapers and they are good for bed wetting in children.
A size 7 Gentle Steps or most other size 7 diaper should easily fit a 6 year old or even likely a skinny enough 10 to 13 year old.
It all depends on how big the child is as some kids are skinnier and smaller built than others.