A hardware wallet is a good choice. Create an address for each one of your children. You can even name those addresses with their names. Give them the private keys as soon as they reach 18. Keep the seed phrase safe and offline all the time. You're saving for your children.
Also, since this is for the next 6-10 years, you might want to focus only on Bitcoin. Avoid altcoins as it could mean they're already worthless by the time they're given to your children.
This is exactly what a lot of people have been suggesting about having a hardware wallet to store money in bitcoins and like a savings fund for the OP's kids, I think this is the best way to stay consistent with investing early.
About the seed phrase, however, this must be learned by keeping it very safe and no one should know except later after the adult child can give it, just store it in a place where no one else knows.
The 6-10 year term is a good term, it should still be in bitcoin and not recommended for altcoins because it will kill your investment when choosing altcoins.
Choose bitcoin and do DCA much like saving