Saving for a very long time for another person might be risky cause if that person isn't even born yet and you still have to wait 18 years after they are born then bitcoin might be already dead.
Unless you manage it for them ofcourse then it's a good option to do cause you could sell if needed.
Not sure why would you need to wait 18 years when you can still secure it while you're still alive. You can make it as a "last will" to your kids/grandchildren. You don't have to wait 18 years because I'm pretty sure you're still alive while they are at that age. You just have to continue doing what you're doing with Bitcoins if possible so that they will have more Bitcoins to use in the future. But yes, 18 years is such a long time and Bitcoin could either be dead or skyrocketed at that time.