Saving Bitcoins for your children is a great idea, passing it to the next generation, I applaude you man. Now for the storage I would recommend a hardware wallet, it is not offline so it is safe from hackers, keep it in a vault or a serucity box in a bank so it is really secured. Include all the ncessary details that your children will need to access their Bitcoins and also entrust all these to the mother of your children, I am sure she can be trusted because both of you would only want the best for your children's future.