There are many different approaches to this, but I have come to a conclusion about Shamir's secret sharing which can be an effective way to this type of issue. Let us take for example, that you have 4 children, you can use the Shamir's secret sharing cryptographic algorithm to use the seed phrase to generate four secrets, the secret can be set to have 3 shares, which means 3 secrets is enough to reconstruct the seed phrase. You will have to let your 3 children know about this, educated them what bitcoin and cryptocurrencies generally is all about, then give them their own secrets. In case any thing happen to anyone, the three remaining can reconstruct the seed phrase. Also, for more ease of recovery, the shares can be set to 2 instead of three, I'm which only two secrets will be needed to reconstruct the seed phrase or private key.

But, also another approach which is the best is the use of multisig, in which M out of N users can be able to sign a transaction, so out of 3 children, you can set it to 2 children that will be able to sign the transaction, so any of the 2 children will be able to unlock the funds in case of anything. This is one of the best approach, but fathers do not want children to know their assets until they leave the world, but multisig can not offer such, your children will be able to know the assets on the multisig wallet unlike shamir's secret sharing when children will only be able to use the shares to reconstruct the seed phrase or private key, if they are loyal in the family.

https://medium.com/shiftcrypto/prepare-your-bitcoins-for-your-heirs-dfc537811df2
This is a pretty detailed guide which covers most of OP doubts. Surely, whenever we die we no longer have access to our beloved keys: for this reason it is of extreme importance to make sure that somebody we love will be able to access our btc stash. If we do not make sure about this our stash is gone forever.

I will advise this to be the simple solution to what fear op has raised and concern to every body. This is not only bitcoin keys though, it is concerning all cryptocurrency coins. We can let our trusted people know about the blockchain and cryptocurrency for at least the sake of how to unlock our wallets. This will help to ensure that our investment and sweat are not lost to the wallet and blockchain.

this sounds scarry  . its like a dead body that have never been discovered and didnt bring properly to his last resort . bitcoins that are stuck in a wallet can haunt too ? jk ;")  .

op you look so worried , do you have lots of bitcoin stored on your wallets ? you should act and do whats best for them . you already have couple of suggestions like giving the keys or other login details to the closest people around you  .  anyway theres a topic that i have read before where the wallet or the address can send amounts automatically to the set address . why wont they implement same thing  where the funds where automatically sent to the set address if no activity was found after some time or based on the time the user have set .

This is fun question and many people do ask it when they first time get struck with it!

Well lots of research has already been done on this and according one of the article there are around 4 million bitcoins which are already lost in the time. Nobody knows if they will ever become available for use. They are in the circulation but not used since they are locked out somewhere on the hard-disk, or the biggest holder like Mr. Satoshi has most of it and it's still ambiguous if we will get it or not.

Actual Lost Bitcoins in the Time.

This is ridiculously huge amount of money that's stuck.

Surely your will be locked out into this pool as well. 

Obviously it will be lost forever, that's why you need to have some plans or contingency. As the saying goes, you are your own bank, so you need to take care of it and somewhat learn how to recover it in case of death.

You can read a medium post here: Letter to Loved Ones: a template for your crypto estate planning. It is an old article, but it is still applicable at this time.

What if Satoshi was dead? I hope he is just fine and enjoying his life right now.
He possesses the most bitcoin for himself, that estimated Satoshi Nakamoto has more than 1 million bitcoins, claimed as 'lost Satoshi Coins'
While we agreed that 'lost bitcoins' make the rest of the coins worth slightly more, there is still a doubt.
it is estimated that more than 4 million Bitcoins are lost forever, and there are many stories of how hackers have been trying for years to crack a bitcoin wallet that contains lost bitcoins, which cannot be accessed. Therefore, the Forum Leader (a person in charge) Theymos ever stated to do soft fork to destroy 'Satoshi lost coins' in order to prevent disastrous monetary (bitcoin) inflation.
However, Theymos stated he never said any such thing, the idea is not to destroy coins, but to prevent lost and insecure coins from being stolen. > https://bitcointalk.org/index.php?topic=1469099.msg14823504#msg14823504

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Bitcoin gives death a unique challenge. As Bitcoin exist only in virtual form. If you pass away without leaving the private key to anyone, your bitcoins are effectively unavailable to anyone.

It would be stay on your wallet and nobody could access it or send it.
This is also the reason why I let my family know that I am earning through Bitcoin and I have some savings on it so if that time comes they would only need to seek help from someone who have knowledge on how to access it or I would leave some note and guide how to use it.
If you want to keep it a secrete to your family you could always create a note or guide on a usd flash drive that would be given to them if something happens to you.

If you don't tell your family that you have bitcoin, no one will know about that, and they will not be able to use your bitcoin for anything. Your bitcoin will be store in the wallet without anyone who can access the wallet. I hope you can tell your family about bitcoin and how they can access your bitcoin wallet to use it for their own good. If they can achieve success to make more money from your bitcoin, they will thank you, and they will not forget you even if you already died.

Maybe you can teach them about bitcoin, how to use bitcoin, and how to secure the bitcoin in the wallet to know the basic things to use bitcoin. One day, if you die and tell them about your bitcoin, they will use it as your legacy, and they will have the opportunity to continue your plan to grow the bitcoin amount or anything you want to make it.