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.
It would be risky since we dont know what would be the track or development of bitcoin in running years to come and you were right if bitcoin would be dead that time then you r hard earned and saved bitcoin would be useless and cant be used by your future grandsons and children.