If you were a "non-filer" who did not get the $500 stimulus for a child under the age of 17:
https://www.irs.gov/coronavirus/non-filers-enter-payment-info-here
If you filed an actual tax return and somehow did not receive the $500 stimulus for a child under 17, then you can get it when you file your 2020 return next year. The IRS has not provided a remedy for people who did not get it after filing their 2018 or 2019 tax return.