My understanding was that the stimulus is $1200 per adult, and $500 per child/dependent.   So, husband + wife = $2400; 6 kids x $500 = $3000.  $2400 + $3000 should be $5400.  Sounds to me like they only know about 5 of your kids. 

Did you have one within the last 12 months?  Did you not file 2019 taxes yet, and so they used 2018 tax return information?