So I try to do special things for those kids turning 8 and those that are turning 12 and leaving to go to Young Men and Young Women. For the 8 year olds we always sing "I Belong to the Church of Jesus Christ" very exuberantly to welcome them as the newest official members of our church and congratulate them on the choice they made to be baptized.
For the 12 year olds we use the song "You're Happy Birthday" but sing it like so!
"This month is such a special one it's goodbye time for you.
We'd really like to celebrate this happy day with you
John-ee-ay and Jenny-yo (substitute names or first and last name if 1 child)
There's something we must do
We'll sing a song that we all know
Goodbye to you!"
Add it waves or actions or whatever. . .it works great!!