E.B.
I did this. I requested a pass to the gate when my son was flying. The ticket agent was quite rude and asked why I needed to sit with him. She actually asked "if he can't sit at the gate by himself, should he be flying by himself?" I backed off, and he went on without me.
Afterwards, I realized that although I had asked politely, I perhaps had not phrased it well. My dh's father worked for an airline and he has told me how much power the gate attendants and ticket agents actually hold. Upgrades, special treatment, requesting a wheelchair - so much depends on the agent's mood and the passenger's demeanor.
We once witnessed a very pompous passenger who was demanding an upgrade, and upgrades were available. The gate attendant denied him an upgrade. We watched her quietly go over to a person who was patiently waiting, with a pleasant smile, who had done something helpful for another passenger (a random act of kindness kind of thing) and the gate attendant called her up to the desk and upgraded her.
So my advice is not to even imply or reference any potential difficulties. Don't say "I'm not worried that something will happen, but..." or "he's very young and is flying with his cousins" or "he's never flown before" or "he's a little nervous". Dress nicely, be extra courteous, and simply say something like "may I please have a gate pass so that I can sit with them before they board". If they deny you, don't argue or plead. They literally can determine that the child is not fit to fly or that you or the child will be a distraction. If they ask you why you want to sit with them, just respond by saying "just so I can be of extra help if they need it, although I know they're all very well prepared for the trip".
Prepare your son for the fact that you may not be allowed to the gate but that's not a bad thing. It's part of the adventure. We play games by people-watching. We try to imagine where people are going and why.
And if I can pass along one piece of advice, from someone who has flown around the world several times, whose kids have flown to other countries and continents, the most important thing to remember when flying is (in my opinion): keep your important items in one place. By that I mean, have one secure pocket in your jacket or backpack, or one secure traveler's pouch around the neck or waist, and when you receive your boarding pass, or get your ID back from the TSA agent, or when you have to take your wallet or passport out, put them back in their designated spot immediately. Most things are lost not to theft, but to randomly stashing things every where, in a different spot every time. Kids put their boarding passes in the magazine they're reading, they are in a hurry and stuff their ID into some weird pocket, they jam their passport into the place where their water bottle normally goes, and sure enough, they lose them because they've forgotten where they put them last. Take the extra 10 seconds to put the documents/ID/boarding pass/passport back in that spot no matter what.
