How could a minor send money from my Apple Pay and overdraft the account?

From Apple Pay, they can't. Apple Pay has no money to be sent or any way of sending money from it.

Apple Pay is just a payment conduit used to pay at stores using tap to pay option or on Websites that support it.

Do you perhaps mean Apple Cash?

Money can be sent from Apple Cash or an associated debit card to other people through the Messages App, but since its a debit card either way, it should not be possible to over draft either of them. They have the amount they have and no more. Unless you have it set to auto fund Apple Cash when nearing a low level threshold you set, but even then, the limit would be your bank account's limit which should not be possible to overdraft unless your bank allows this.

Seems like some information and context is missing here.

click here ➜ Send and receive money with Apple Cash - Apple Support

Perhaps if you clarify what exactly happened, or what exactly you are seeing that tells you something is over drafted we can get to the bottom of it.

Apple Pay on its own cannot be over drafted.

A Credit Card added to your Wallet app may be over drafted, but that would require access to your device and your face / thumb or passcode to confirm the purchase.

Without specifics as to the exact purchases everything stated is a possibility. One common cause is game token purchases. Youth play online games and keep purchasing tokens to play and advance levels. Like many merchants, Apple does not instantly charge the Apple Account but accumulates multiple charges and charges the debit card a few days later. This can result in an account being overdrawn.

Does this minor know your passwords?