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.
BTCSo 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.

Yes you must save bitcoin for your children. You can invest in bitcoin now because time will come that the value of the bitcoin will become higher and bitcoin will go higher and higher that is why it is essential and would be helpful for you if you save and invest in bitcoin today. It is smart and legit.