The total child tax credit amount is $3,000 per older child (aged 6-17) and $3,600 per younger child (age 5 and younger). The advance portion of the credit is half of that, with the balance paid in a tax refund next year. For example, a parent with two children who will be 6-17 or under at the end of this year can still get $3,000 in advance payments between now and the end of the year ($3,600 if the two children will be 5 or under at the end of the year)!
But there is an October 15, 2021 deadline for using the new, easy-to-use GetCTC site (EspaƱol) (a partnership between Code for America, the White House, and the US Department of Treasury) and the IRS non-filer portal to sign up for the advance payments. Assuming that a low-income family does not have a reason to file a "regular" tax return, using one of the portals is the best way to get the advance payments.