I couldn't agree more paxboy/rmac. It would probably be every man for himself in this situation!! You would look after you and yours as the survival instinct would kick in.
This is exactly why the CAA issue "guidlines" which most airlines will call "rules" if required to enforce them on boarding re the seating of children near their parents. For those that don't on the day and it's a problem for you personally i would start quoting those guidlines which any parent can find on the CAA website.
It would impede the flow of an evacuation if Dad goes to search for junior who is seated a few rows away and he would. I know as a Mum I would!
Nobody (Pax or crew) unless they have been there knows how they would react in this situation. What I do know (from experience) is the pax will probably not be studying their safety cards and a lot of airlines include this as part of their drills.
The post by Tightslot is spot on re the realities on board in this situation for pax with restricted mobility