The shelf life of powder in the original can will be at least as long as the shelf life of loaded ammo. A lot of WWII surplus ammo is perfectly fine to shoot today. If the can is properly sealed and kept in a moderate climate like you described, your children will still be able to use it when you have turned to dust.
Good question, I've actually wondered that myself. Problem solved, when I start reloading I'll just keep reloading as much as I can without having to worry about the shelf life. Also, when your finished reloading be sure to put the powder back into the bottle and not leave it in the powder drop.