That's not ruining their life.
One of the richest men in the world has an incredibly uncommon name.
If anything, this will open up opportunities to the child that would have been impossible otherwise.
So what's to stop people from renaming the kid after a month or two? (is it a difficult process?)
I would assume that Bethesda would have already thought of that, and will enforce a contract enabling them to revoke the prize if the child's name is changed.
I'm not certain, but I'd think that being able to change your child's name is a legal right, so Bethesda can't contractually prevent them from changing it, but they have complete control over the prize. If it's a perpetual license key, they can easily revoke it.
That said, I wonder how they would react if the kid changed his name himself when he reached 18. Would Bethesda still care about it enough to revoke it, or will they let it pass since he never had a choice in it?
However, having a single master key that unlocks every Bethesda game wouldn't sound like a good idea. All it takes is one persistent hacker to reverse engineer the security in the game to unlock it for everyone. The only way they could get around that, afaik, would be to never use the same keys for Steam and retail/direct download packages, or just send the kid a new key for every game.