4,200? That sounds like the Recovery Rebate Credit on line 30 not the Child Credit. Do you have 4200 on line 30? The 3rd Stimulus was 1400 per person. 6 people x 1400 = 8400. Did you get 8400 last year starting in March 2021? They sent out letter 6475 telling you how much they sent you. They sent a letter to EACH SPOUSE for half. I think you only entered one letter for 4200 and the other half showed up as missing on line 30. Line 30 is only if you didn't get the full amount or qualify for more.
Double check your bank statements and check your IRS account (both accounts if married) to see what they sent you.
https://www.irs.gov/payments/your-online-account
