If you are able to come, please confirm your acceptance to [name] at [address] or call 890-123-4567 as soon as possible.
Is this add-on fine?
Yes, it is fine, but omit "at" before the number.
This is how my letter would be:
On [date] we are holding a Young People Day [name] church. We would very much like to invite you to attend this event and hope that you will be able to join us.
We will hold two services that day - one in the morning from 9:00 am - 12:00 pm and the other in the afternoon from 2:00 to 5;00 pm. Also during the afternoon there will be some games/activities such as game A, game B, and game C.
If you are able to come, please confirm your acceptance to [name] at [address] or call 709-1234 as soon as possible.
We will look forward to hearing from you.