Letter from irs

 we got a letter from the IRS stating that we got direct deposit and we got the $2,400 married couple we did not get the $500 per child and we have three children under the age of 18 this letter that I got from the IRS 15 days after their first deposit only let me know about the married couple stimulus and how it was sent to me. How can I get ahold of the IRS and let them know that I did not receive the $500 per child someone told me this letter that I will get so let me know what to do if I did not get the $500 per child this letter does not say nothing to that nature how can I go about getting a hold of the IRS to let them know that they are missing the $500 per child to me