My granddaughter bought stuff on Roblox without me knowing, can I be refunded?

Try this link for possible refund...

Request a refund for apps or content that you bought from Apple - Apple Support

She could not have done so without the credentials for your account or you setting up her face or Touch ID on your device and allowing that for purchases.

Apple has been known to give a one time refund in some circumstances like this, but you are responsible for securing your account.