If arrivals and departures are in a mixed area, sometimes you can do a back to back connection to the same aircraft without passing immigration, customs and security. Typically, this is only possible if you are within some common travel area such as a domestic flight or some intra-EU flights. Otherwise, you would normally have to pass immigration and re-clear security, especially on an international flight. The answer in your case depends on origin, destination and airline.
You second question is unclear. You state you're passing immigration twice. It seems you are departing, pass security, don't get on the flight and exit again? This depends on whether there are exit controls, where you are going, and whether the airport configuration allows you to exit without passing immigration. For example, if you had a boarding pass for a flight from JFK, you could pass security, and then exit straight away. You'd only pass immigration if you arrived on an international flight.
Edit: After your clarification, another option in some cases, is to get a gate pass. This allows a non-passenger to go airside, for example, to assist a dependent passenger. This is only allowed and possible in some airports.
My answer is generic and doesn't address your specific itinerary. I'll see if I can find out, otherwise someone else might provide specific information.
Update: Here's an old post from 2008 on Flyertalk:
Turnaround at Phnom Penh (PNH).
Transit desk unmanned and transfer security also. No OLCI or return
check in so I had no BP. Ground staff took my ticket and passport and
went away to get it issued landside. They arranged for someone to open
up transfer security for me. Once airside they escorted me into the
contract lounge to await my boarding pass. Documents all returned as
boarding was 3/4 complete.
No need for me to enter country (requiring visa on arrival), although
I had photo and cash just in case.
That shows there's some possiblity of doing step 1, but certainly doesn't look guaranteed.
I can't find anything about getting a gate pass in PNH. This isn't a very common thing so I would not be surprised if it's not possible at all there. People decide not to fly on many occasions, so my speculation is that the mother could just enter, drop your son off to you and then inform the ground staff that she no longer wishes to fly. I don't know whether she would need a new visa in that case.