our tax preparer didn't give us all the credits we were entitled to help

[DDaccountant]

I know how to do taxes now, and checking a prior year tax return they did for my parents they gave them less than half what they should have given them for the EIC, they only gave us EIC for one child, even though we had 2 qualifying children, so the refund was about $2300 less than it should have been. Thee problem is that it can't be amended because its been more than 3 years. So what is the most and least can we do at this point? the minimum I assume is have them refund the tax preparation fees, But can we sue them? or do something else?
They suck, they didn't even gave us any advice, and for three years that I went to school part time they could have told us that all needed to do to get the full EIC was be full item student