I didnt receive all of my stimulus money. I received $1200 for myself but I didnt receive the $1000 for my children. How can I figure out why?

If your children were under age 17 and you claimed them as a dependent on the tax return (2018 or 2019) that the IRS used to calculate your stimulus payment, you should have received a $500 payment for each of them. If they qualified for it and you didn't get it, you should receive a letter from the IRS withing 15 days with information on what to do about it. 

The IRS will be sending you a letter in the mail 

https://www.irs.gov/coronavirus/economic-impact-payment-information-center

For security reasons, a letter about the Payment will be mailed to each recipient’s last known address within 15 days after the Payment is made. The letter will provide information on how the Payment was made and how to report any failure to receive the Payment.

This also happened to me. I recieved the irs letter but there was nothing on there about what to do if you didnt recieve the correct about.