Your answer is on page two of this document:SM120X_UserGuide_EN.pdf (shareddocs.com) See case #2. In this case the pull to activate handle closes the connection between the Gray 9V power out of the Kiddie module and the Red interconnect cable Forcing all interconnected smokes to alarm. So you just need to replicate that if you want to trigger the alarm.
If you shunt across the gray and red coming out of the kiddie - you get an alarm signal on red. so - you need a switch AND a sensor. SO because of that, I’d use something like a Zooz Zen16 in place of the contact sensor. You can use it to trap the alarm by connecting one of the switch inputs Orange and Neutral from the Kiddie module, then have it activate an alarm by connecting one of the switch inputs to Gray and Red. There may be some other config - but should work… Jimmy @Automated_House knows more about the Zooz devices and I know for a fact he at least used to have a ZEN16 - maybe he could offer some suggestions on that config.