A 9 year old child would wear a size Youth Diaper.
Although some 9 year olds may fit into a size 7 or size 8 diaper.
If the 9 year old will not fit into a size 7 diaper or a size 8 diaper then they would need a Youth diaper.
You can buy Youth Disposable diapers that tape on like toddler diapers but they are sized up large enough to fit older kids and teens.
The size 8 Youth Diaper from Cuties which is size 8 Boundless diaper is designed to fit youths that weigh up to 58 lbs so the 9 year old may be able to fit into them.
It depends on the weight and build of the 9 year old on what size diaper they would need.
The tape on Youth diapers are better for those with bed wetting and incontinence issues than pull ups are.
If your kid is a heavy wetter then the pull ups cannot hold up to the wetness.
The tape on diapers can hold up better than pull ups and keep the pee inside.
Attends makes an Attends Youth diaper as well which is a good Youth diaper for older kids who won't fit into toddler sized diapers.