I'd suggest presenting situations that actually involve their attempt to rescue the boy. Whether you do that in the pre-planned way that [MENTION=177]Umbran[/MENTION] suggests (I seem to recall that Umbran GMs a lot of GUMSHOE) or the improv way that I personally would approach it seems secondary. The primary thing is to actually engage your players with the thing they care about.
To be clear - the action/rescue scene can be totally improv, no problem. I do run a lot of GUMSHOE, as the main game I run at home is Ashen Stars - and there what I plot out is the mystery. Folks tend to overestimate their ability to create a sensible mystery by improv - when you just make up mystery elements on the fly, without time to *think* about how they fit together, you tend to have plot holes and an incoherent mystery. The action sequence at the end where one typically deals with the BBEG, those you can totally improv.