How to set some calculated value to child item type.

Hi, I am trying to set Property weight to the child part which is in PART BOM relationship. when I alert the name of childPart and classification it gives me correct result, but when I going to set new property value like this, childPart.setProperty("_rm_weight", weight); var res = childPart.apply();   this property will get set to parent Part. How Do I set "_rm_weight" property.
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