My 8 year old son wears Pampers Baby Dry size 6 for bed wetting and I've caught him masturbating in the diaper in the morning when he wakes up wet.
I told him it's okay to do it and not be embarrassed.
His 2 year old brother also sometimes masturbates in his wet diaper.
I've also seen my 8 year old nephew who has autism masturbating in his wet Pampers Diapers when I was at his house before.
He seems to do it a lot.