var stepchild=parent.children.find{ it.id==xInt };
stepchild.metaClass.birthMom=biologicalMothersName;

parent.children.each{child->
    //when it gets to stepchild no metaclass property birthMom exists....
}

var stepchild=parent.children.find{ it.id==xInt };
stepchild.errors.rejectValue('parent',"Not biological parent");

parent.children.each{child->
    //when it gets to stepchild no errors....
}

stepchild.metaClass.setAttribute(stepChild,'birthMom', biologicalMothersName)