"and received $5600 in recovery rebate stimulus for those 2 dependents"

It's $1,400 each. Do the math. That's 4 people. Who is the missing 4th?

Are you confusing the Child Tax Credit advance with the Stimulus/Recovery/EIP advance payments?

"should have only gotten $4200 in stimulus"

$1,400 X 3 = $4,200

"did they take into account that the dependency status could change from 2020 to 2021 due to age?"

You do this. You remove a dependent, you add a dependent, and if you do a return for someone, you also make sure to checkmark if they can be claimed as a dependent by another. That question is not "are you being claimed?"