Roblox Unauthorized charges that keep reccuring California

Like almost every kid in the country, my eight year- old grandson wants to play Roblox every day. I once bought him some extra game points for FIVE dollars as a reward for doing his chores. Ever since then they keep taking money from my account - to the tune of HUNDREDS of dollars! I am a senior and very ill and disabled, living on disability, raising my grandson. The unauthorized charges from Roblox take my $$$ , sometimes cause an overdraft, and cause my card to get blocked, leaving me with NO money for food and medicine until I go through yet another long phone cal with the bank. I have changed my Visa card FOUR times but it hasn't stopped the charges! I am beside myself! This is causing so much stress and difficulty! 

I am sure they are doing this to thousands of people. This abso!utely should be STOPPED! Needs to be investigated and there should be a class action suit!