AFAIK, a US passport is required of US citizens when returning to the US by air. It is possible to return to the US using a birth certificate or a registration of birth abroad for minors under 15 yrs when crossing the border by land from certain countries, specifically Canada and Mexico.

See https://help.cbp.gov/s/article/Artic...language=en_US

So that may be an option to look into (ie travel to Canada and have the US parent bring the child back to the US ) if obtaining a passport in these covid times is proving to be a challenge. Once here, they can get a passport locally with the required documentation.