Yes it is okay to ask a babysitter to diaper an 8 year old boy.
The 8 year old boy should be wearing diapers all the time anyway and they should be diapered by their mom, dad or even babysitter.
When I was 8 years old my babysitter also made me wear diapers and she diapered me and it was great as I love wearing diapers and loved wearing diapers then too.
I was so happy when my babysitter came over because I knew it was diaper time which I enjoyed.
I remember one time when my parents were gone for a month on vacation and they went without me but I was happy because I knew I would get to spend the entire month wearing diapers all day and all night.
During that summer month I was babysat at our house my babysitter put me in diapers and changed my diapers every 3 to 4 hours and I loved wearing the diapers.
She even took me outside in our fenced backyard wearing just a diaper or just a diaper and shirt and I played in the sprinkler and went on the swing and down the slide wearing just a diaper and shirt.
If I babysat any kid even an 8 year old boy they would be wearing diapers too and they would not be wearing any pants or shorts over the diaper.
Right now I'm wearing just a diaper and shirt and it feels great.
I was dressed like the diaper boy below.