Something like this?
Dear Steam,
An unfortunate problem with my Steam account has arisen. It appears that yesterday evening my AGE son come into possession of my (his mother) credit card and ordered WHATHEORDERED without my permission. His purchases have totalled £TOTAL. If it is possible, I would like a refund to my account. I appreciate your help in this matter and apologise for taking up your time.
Yours sincerely,
Mummy Spyder
Dear Steam,
An unfortunate problem with my Steam account has arisen. It appears that yesterday evening my AGE son come into possession of my (his mother) credit card and ordered WHATHEORDERED without my permission. His purchases have totalled £TOTAL. If it is possible, I would like a refund to my account. I appreciate your help in this matter and apologise for taking up your time.
Yours sincerely,
Mummy Spyder