Help! Can't find our Roblox account billed from Apple.com

I'm writing to the Community in hopes that can point me to the solution of this problem.

My daughter has a Roblox account for her daughter (age 7) who loves the program. A large number of charges have appeared on my daughter's Visa account from Apple.com for purchases within Roblox, used to buy things within the program.

My daughter is incapacitated right now and she has asked me to help straighten out the account. I want to change the password and add the optional 2-factor authentication.

Support from Roblox gives instructions on how to do this. One has to go to the original account,

I don't know how to find the original account in order to follow the instructions.

Any ideas?