So do kids voluntarily enter into an agreement with Muslim parents to live under Sharia law? Settling family disputes inevitably involves children who have no say in their situation. What you suggest is akin to allowing people to sell themselves into slavery. There are some things you cannot agree to under contract and maintain a legal system based on western values.
Please. All legal cases involving minors are by definition resolved by others without their consent.
Are all children in western society slaves because they are unable to execute contracts on their own behalf until they're 18?