if she tries keeping the kids and not returning them, you can file a deprivation of parental rights charge against her
it is a felony
it is not a criminal court thing so the police don't usually enforce it but they can call her and let her know that it is a felony or stop by her house and let her know that
I know what it is like to be the new fiance that the ex finds crazy made up faults with...too bad for your ex...it is jealousy that you are happy, your kids like her, and she is no longer queen bee.
I am glad you found someone that makes your heart light!!!