Hello . My son bought many Roblox gift by my credit card and I want money back because it’s around 300$, he’s 10 years old

This is a user to user only forum. We aren't Apple, nor can we refund anything.

Here is a link about how to request a refund from Apple --> Request a refund for apps or content that you bought from Apple - Apple Support

You might be able to request refund for items purchased through Apple (apple.com/bill appears on card statement) here: Request a refund for apps or content that you bought from Apple

However, if it went through Roblox, you will need to contact them.

To prevent unauthorized purchases though Apple from happening again, you can setup Family Sharing. During setup, enable Screen Time (parental controls) and Ask to Buy (to require approval before purchase), then login to the new Apple ID on the device.