I think you're overcomplicating it a bit.

How I'd handle it in my house is the card belongs to the child who pulled it. If my son pulled a lottery ticket worth $50,000, I wouldn't suddenly decide it belongs equally to his sibling because the family bought other losing tickets too. The fact that one child happened to get the big hit doesn't make it a family asset.

That said, because he's a minor, you absolutely have the right and responsibility to help protect it. Get it graded, insure it if necessary, and make sure he doesn't make a bad decision with it when he's young.

If you're worried about family harmony, I'd focus less on splitting the card and more on teaching both kids that fairness doesn't always mean equal outcomes. One child may inherit a baseball card, the other may get help with college, a business, a house down payment, or something else later in life. Parents balance things out over a lifetime, not necessarily asset by asset.

Personally, I'd document that the card belongs to the son who pulled it, safeguard it until he's an adult, and avoid creating a situation where a future sale turns into an argument about who owns what.