To file an amended return with a claim for refund, you must file the amended return within 3 years of the original deadline, or within 2 years of the date you actually filed, if you filed after the deadline. If you never filed at all, the claim for refund must be filed within 3 years of the original deadline. For most taxpayers, that means that 2019 and earlier are closed for amending to get a refund, but an amended return could still be filed for 2020, 2021 or 2022.
Whether you will get a tax benefit is very complicated, depends on many factors, and you didn't give us any details. If someone else claimed the child as a dependent, even if you were the person who was legally entitled, making a duplicate claim now will require the IRS to investigate you and the other taxpayer to determine who has the correct claim.
We would need to know a lot more details. What is your relationship to the child, when was the child born, where did they actually live (regardless of any custody agreement), who provided support, were there court/custody orders or agreements, did someone else claim them as a dependent, approximately how much income did you have and what kind (working or something else) and so on.
