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When Richard died, she owed the money to his ESTATE, so you need to file to open probate and you need to then get named as executor/administrator of the estate and then as administrator you can sue her on behalf of the estate to recover the money that she owed Richard at the time of his death.
It is NOT TRUE that she no longer owes child support arrears because he died. She might not owe future support, but she DOES OWE the arrears and the estate has to be the one to sue her for those and the money would then be distributed to the heirs of the estate, which would be you and his child.
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