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.
Haha is an interesting advice, truly in most family or every family there is always one prodigal son that always object to almost everything in the family, so when a father discover he has that kind of son he make sure everything will be well stated on the will to avoid problem among themselves but sometimes will could be tempered if the lawyer is not trustworthy and it will become a game of wealth were the children will be fighting to acquire everything, though it happens more on polygamous family were a man married many wife and has many children the hatred against each other will be extreme.
So just like you mentioned about using USD to record voice message or video, I have seen were a man backed up his will through a video recorder and he made four videos, giving the lawyer one and writing on the will the all his wife should check on a particularly place in his room they will see there various messages to proof the authenticity of the will.