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Hello @Huy Phan
I understand how frustrating this is, especially since it is preventing you from using Minecraft multiplayer.
A child account cannot remove itself from a Microsoft family group. The organizer who originally granted consent must remove the consent and then remove the account from the family.
Since the organizer account is inaccessible, first try recovering it using the Microsoft account recovery form. If recovery is unsuccessful, contact Microsoft Support while signed in with another accessible personal Microsoft account and explain that the child account is stuck in an orphaned Family Safety group.
Please note that this is a public user-to-user forum. We cannot access accounts or manually remove family memberships here. Also, for your privacy, please do not post your email address or other account details publicly.