To get your big child to wear diapers show the child pictures of other older kids wearing diapers.
You can find older kids wearing diapers pictures online and on the website https://imgvek.com
Here's a few pictures of an older kid wearing diapers.
Here's a few pictures of a boy named Jarod who loves wearing diapers.
You can see more of his pictures by clicking the pictures or by clicking the below link.
These pictures were taken in 2001 so he's likely in his 30's now.