There are two types of dependents, "Qualifying Children"(QC) and
standard ("Qualifying Relative" in IRS parlance even though they
don't have to actually be related). There is no income limit for a QC but there
is an age limit, a relationship test and a residence
test.
Only a QC qualifies a taxpayer for any refundable tax credits. So, if your tax has already been reduce to zero, by the credits for your 4 kids, claiming your Mom will not get you anything else. Otherwise, your mom, as a dependent, would qualify you for another $500 tax reduction (tax credit).
Prior to 2018, dependents (QC or QR) qualified you for a $4050 exemption (deduction from income) before tax was calculated. This is no longer the case. For a QC, the deduction was in addition to the credits
