When we recall a mortal sin, it should be confessed, however long ago it happened.
As for childhood, that is a prudential judgment for a pastoral need. I personally recall being a perfectly wicked child who used to do very bad things, and be willfully disobedient, knowing that it was wrong to act that way. I think children have much better developed consciences than many adults are willing to credit them with.
If you recall a thing that you did, even as a child, with the full knowledge of wrongdoing at the time, and it weighs upon your conscience, by all means bring it up in confession. Stealing that ice cream bar when the Good Humor man wasn’t looking was a sin, albeit a venial one. If it bothers you, give it to the Keys.
A lot of priests will not want to hear this. So preface it with the statement that you have only now recalled this thing, and that at the time you did it you were fully aware that it was sinful (if this is true, of course). But don’t go confessing childish mischief of which could not have foreseen the consequences. I’m thinking of the broken window pane from an errant baseball. Now, if you had been warned not to play baseball next door to Mr. Jones’s plate glass window, and you did it ANYWAY . . . Well . . . .
Keep in mind that these things would be confessed in the context of childhood, and make it clear that you are confessing them out of your own pastoral need and not because you believe that the Lord will send you to hell for a childish prank.
Hey! How would you like to be the 9-year-old boy who accidentally started the big wildfire in California?
When I came into the Church and made my General Confession, I felt very keenly the sting of conscience over things I had done in childhood (those mudballs in Mrs. Zaccharelli’s laundry when I was 8

). I realized that the sins of my childhood, far from having been inconsequential, had actually seeded the sin-history of my adolescence and adulthood. I made a summary of the nature of my childhood sins: temper, disobedience, selfishness. But that’s kind of a different thing from what you are asking.
Use your judgment – presuming that you HAVE judgment. On an internet forum, that cannot be assumed, but I’m giving you credit.
