Change a 'child component' expression in Fusion 360

change_child.py

#Author-
#Description-

import adsk.core, adsk.fusion, adsk.cam, traceback

def run(context):
    ui = None
    try:
        app = adsk.core.Application.get()
        ui  = app.userInterface
        fd = adsk.fusion.FusionDocument.cast(app.activeDocument)
        data_file = fd.documentReferences.item(0).dataFile
        child = app.documents.open(data_file)
        param = adsk.fusion.FusionDocument.cast(child).design.userParameters.itemByName("side_lengths")
        param.expression = "5 m"
        child.save("this is different!")

    except:
        if ui:
            ui.messageBox('Failed:\n{}'.format(traceback.format_exc()))