Yes I would babysit a 6 year old boy who is still in diapers.
I would babysit any kid of any age who wears diapers full time or any time and I would be glad to change their diaper as well.
If I babysit a kid they will be in diapers at my house and not allowed to use the toilet.
Here's some cute diaper boys that I wish I could babysit.