Once USCIS approves the child's SIJS application, do they automatically receive a green card?

No, the SIJS approval by itself does not provide any benefits. Once the applicant's SIJS application is approved by USCIS, they must then complete the final step in applying for a green card. Note that children from Mexico, Guatemala, El Salvador, and Honduras may have to wait to submit their green card application because they are subject to visa delays based on the visa bulletin that is published every month by the Department of State. In these cases, the wait can be for many years, as the applicant must wait for an immigrant visa to become available.