Stimulus Check for 9 Year Old

My 9 year old son did some acting work back in 2018...only making <$1000.  I did file a tax return for him for 2018.  I received $3400 Direct Deposit from the Covid Stimulus for me, my wife and both of my kids) a few weeks ago.  Yesterday I received a Covid Stimulus paper check for $1200 for my 9-year old because of the 2018 tax return I filled out for him in 2018.  I cannot contact the IRS (limited staffing) and a Google search did not find any similar situations.  How do I determine what is legal here?  How did he even get a Stimulus check if he is only 9 years old...I thought the eligibility age was 18 and over?  If I cannot contact IRS how do I get this resolved?