I made a paper wallet for my son and I deposit a smaller amount of Bitcoins there every month. I kept it in a secret place that only me and another person knows.

Like every other investment such as real estate, stocks and other forms of investment we know that you can invest in that kind of things and then you get something like say a Certificate or a Binding Document as a proof that you are part of that investment, now that of bitcoin all you have is just your address, your password, key and seeds as proof of the ownership of the wallet.

BTC

So now if one is considering buying bitcoins and saving for his/her children in the future is there any way to do that with bitcoins apart from having to write down your details somewhere. I'm asking because with that kind of stuff if no one knows your seeds or password no one can get the coins.

I'm not sure it's good idea.
Everything changing very quickly on Internet.
It is impossible to predict how the situation with bitcoin will develop in the next 20, 30 or 40 years..
Some very rich people, like Bill Gates, didn't give money to their children but knowledge.
We can learn something from it.