Your kids don't have to be under 18 for the EIC. They do have to be under 17 for the Child Tax Credit (CTC). So, it's not clear if you're asking about the EIC or the CTC or both.
How much EIC, you are allowed, is a straight look up*. See the 2022 EIC table at:
https://www.irs.gov/instructions/i1040tt#idm140702909059168
The child tax credit (CTC) is limited to your tax liability. The CTC is a non-refundable credit and can only reduce your income tax to 0, It can not help you beyond eliminating your tax liability. But, if you have more than $2500 of earned income, some or all of it is usually given back to you thru the "Additional Child tax credit". That is, part of the CTC may be on line 28 of form 1040 (2021 & 2022) instead of line 19. The ACTC is calculated on form 8812 and is basically 15% of your earned income over $2500. The ACTC is a maximum of $1500 per child (not $2000).
*The earned income credit is first calculated (actually looked up in a table) on your earned income then it is calculated on your total income (AGI). You get the lesser of the two calculated EIC numbers.
