Getting an older kid to wear diapers can be hard.
Most older kids do not want to wear diapers because diapers are seen as being for babies and the older kid does not want to feel like a baby.
However if the older kid needs to wear diapers such as for urinary incontinence then you need to have a talk with the older kid about how the diapers are not just for babies and that many older kids and even adults need to wear diapers.
If the kid won't wear regular diapers maybe have the kid wear pull ups in their size.
Also let the kid know that there are diapers that have no babyish designs such as Gentle Steps size 7 diapers.
The only design they have are a little strip on the front for the landing zone which holds the tabs in place.
My 8 year old son wears Gentle Steps size 7 diapers and they are mostly plain white and he actually likes the little tiger that is on the front.
I explained to him that it's okay for older kids or anyone to wear diapers.
Nobody knows he wears them and he's incontinent and needs to wear diapers all the time.
I taught him how to put the diaper on himself and he feels much better.
Sometimes you just have to sit down and talk to them about the diapers and give it time.
Eventually the older kid may decide on their own like mine did that wearing diapers is better than having wet pants.
If the child is wetting the bed then offer them a pull up, cloth diaper, or youth diaper or even the Gentle Steps diaper and tell them that having a diaper on is much better than waking up in a wet bed.
Or if your kid still does not want to wear the diaper then don't force them but instead put down a washable bedding cover or buy disposable bed mats.