You do not get the stimulus for the other two children unless you claim them as dependents on your 2020 tax return..
That means the Ex has to NOT claim them on his 2020 return, and lose the $2000 (each) child tax credit (assuming the kids are under 17). The child tax credit phases out at a much higher income level (more than $200,000, $400,000 Married) than does the stimulus payments.
