I'm looking for some kind of storytelling or practical effect involving the vanishing candle catered to a children's performance

I think that you are looking for a PRESENTATION. The EFFECT --which is what the spectator(s) perceive(s) IS already "in" the "title".

The "Vanishing Candle", IMO is generally, a "quick" flash trick. Tricks using FIRE are not generally recommended for children's performance. (In Chicago, for example, NO FIRE TRICKS are ALLOWED in schools.)

Years ago, I brought out a candle (unlit) and said it was somebody's birthday today and pointed to a child and said "how old are you today?" and they said "It's not my birthday!"
My mistake, and whipped out a silk, covered the candle, vanished the silk, and said "then let me show you this trick."
It got a good laugh from the kids.

I used to end my Halloween shows with bringing out the candle and after giving a little speech while lighting it, I would wrap a silk with "Good Night" written on it with a rabbit holding a candle wearing a night cap. I had the kids blow towards the candle to end the show then showing the "Good Night" written on the silk. I would have at least one of the grown-ups come up after the show saying how much they liked that. It got everyone to relax and it sort of put a cap on everything.

Awesome idea.

As a quick trick and a funny line.... I've said, "Raise your hand if you celebrated a birthday this year." Naturally, everyone raises their hand.

I conclude with, "Than this birthday candle is for you. Happy Birthday." I then vanish the candle.

Your clever line, motivated MY mind, and, I'll toss in a follow up line.............

"If you think that this candle is too big for a birthday cake, you should see the size of the cake!

Samuel Patrick Smith has a vanishing candle routine for children in his book ‘Big laughs for little people’

https://www.spsmagic.com/products/big-la......e-people

John