I am baffled that someone with a child under the age of 17 would not notice the credit they got was only $500 and not $2000 ... how did they not question this back then ?
Now if you do believe this was a program error then you can put in for an accuracy guarantee claim :
https://ttlc.intuit.com/questions/2566201-what-is-the-turbotax-100-accurate-calculation-guarantee
