Hello, thank you for reaching out to Microsoft Q&A.
So, when you sign in with your Microsoft account (Child account) on the Family safety webpage, do you see her (grandma) account as the Organizer?
https://account.microsoft.com/family/home
If the answer is 'Yes', unfortunately, you will not be able to make any changes to your account because only the user with an 'Organizer' role can remove or manage consent for the child account in the family safety group. If you no longer have access to the Organizer account, you will have to follow the account recovery procedures offered by Microsoft to recover your account.
Also, only the Organizer can change a child account's date of birth to convert it to an adult account. Without Organizer permissions, you won’t be able to make any of these changes.
Reference:
How to leave a Microsoft family group