Not sure there's one sentence... you just do it and share it! I can summarize my approach something like: Hey grandma, look, I put some savings here and I'd like you to keep a key safe.

Have a multisig setup with my grandma and another family members 3-5, giving a key to her, help her feel included in this inheritance process. It's taking years for her (ad other fam members too) to understand why I did it. She sometimes spontaneously asks questions, and she knows all money she sometimes gifts to her' grandchild babies goes there. She don't know how to buy, she don't need to... I do it for her.

Making it available without explanation make her curious to learn more. Also provided kids book for her to learn meanwhile she read them out to her grandchild.

Having elder people participate, but not 100% responsible, I found it help spread awareness around small circles (like families).