Just for the sole intent of your heirs, I really wish that there was a solid and fool proof way to leave legitimate BTC to your kids in some custodial solution. Something that was protected by like a time vault or something. o much out!
With Bitcoin, proof you need is a private key to sign a message, sign a transaction, sweep a wallet, that's all you need as proof of ownership. If you keep your private keys safely, it is against threats and time and it will be reserved for your children as their inherited fortune from you.
There may not be many solutions today, but I am sure there will be in the future, but as you say, most of them will be custodial. But once inherited, your children will be able to withdraw them to their hardware wallets. If you want creative ideas on the subject that can be applied today, I would move this thread to the corresponding technical board, where I am sure they will be able to help you.
It's terrible if you have an account on a centralized exchange, KYC, leave your bitcoins there and award it to your children. You don't know when you will pass away and when your children will inherit it from you. Meanwhile, that exchange can be seized by government, can be hacked and file a bankruptcy or can make a scam exit.
If those fiascos happen, no way for your children to get those bitcoins.