If you received less than you were due, when you file your 2020 tax return, the shortfall will be added to your return.
To get the supplementary child stimulus check payment:
Note: Thousands of college students and high school kids who are older than 17, but are still being claimed as a dependent by their parents or relative on their federal tax return, are unfortunately ineligible for the additional $500 payment based on current rules.
Sorry but that is not the whole story, if read this https://www.savingtoinvest.com/500-stimulus-check-for-dependent-children-eligibility-and-payment you will learn that the IRS had glitches and that is the reason child stimulus payments were not made and why many eligible Americans with underage dependents including my wife and I who have a special needs child were not received 😡