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.

that's a nice and smart move from you, props to you.
Saving something every months (fiat, btc) for our childrens it's the best way to give them something to start with, you never know what's the future holds and if now we are able to save something that's something that everyone should do, think about our childrens!

I would buy an hardware wallet in a security box in a bank (or hidden somewhere safe aka fireproof, waterproof, dustproof, etc.etc.) with the instructions and give all the details to another trusted person, that's it.