If you have a few children, you could try using multisig, but then there's that one of your children dies after you, but before they can share the money.
You could also use a time lock, but then, it doesn't mean you'll time it well enough. You're 50 and you have a 10 year old child, so you lock your coins for another 5 years, but you're still alive, so you keep going, finally you lock it again and die a year later, so your child will have to wait 4 years? That's pretty harsh.
I'd put the instructions and seeds in a capsule and hide it somewhere. I'd give the location to someone that I trust, but make it hard for them to break into it. Let's say you have children and you tell them where the capsule is, but it's under a concrete floor in your living room, so they'd have to use an electric tool and need a whole day to uncover it. They won't do it wile you're alive, but should have no problems getting to it when you're gone.
I think we should begin to think like adults now. As giving ideas and then pointing the flaws out of them are not going to help the OP. And to be honest I do not think OP is going to read all the replies but still he is doing then, he should search for a lawyer or a trust party whom he can give the responsibility of dividing heir in his Childs. But the best practice is (if the children are adults) to divide the heir by himself.
But if they are still child as in your examples then he should hire a middleman or a trust lawyer whom he could give the responsibility of diving the heir equally. But sometimes those middlemen turn up as the biggest scammers and they do not do as they are agreed. So, to avoid that, buy a Hardware wallet and give it to that middleman and tell him it is a USB which contains my last message for my kids. You have to give this USB only when I am gone, and they have become adults. To back this story up and put some extra spice he could buy 1 Hardware wallet and 1 real USB and market USB as 1 and HW as 2 and could say the middlemen that he has to give my Childs the instructions that they should play the video by USB first and in that USB, father could save the Video. Now there are many problems to this method too as if middleman watched the video, he will know what is going on here and that middleman might become greedy. So, find a trustworthy man to handover the heir agreements of BTC.
PS: I think now I began to think like child here. But the real problem is when the Childs are young and not old enough to receive the funds, so the best thing is to lock them even the Childs have to wait for 4 extra years. As it will be a good point for the child as in next four years a halving will came and those BTC will have even more value.
