Hi - we've travelled all over with our toddler and quite frankly, I find that if I say nothing, we get through faster. We've always put the diabetes backup supplies including the PDM through the xray and it's never been a problem. My son simply walks through the screening with no questions asked. I'm always amazed that when I carry 200 extra syringes and don't tell the TSA, they never say a word. In the beginning, when will was 18 months and we travelled, I brought letters from our doctors, all the backup supplies, I would notify the TSA before I put anything on the belt, etc. and overall, made too big a deal of it all. Once I wised up, I simply would bring all my stuff, put it on the belt and let it run through. In the some 2 dozen trips, I think one security guard asked me once if they were diabetes supplies and I replied "yes" and that was that.
My only bit of advice is to bring lots of food for your toddler. My son never liked plane food and it's impossible to find milk on some flights now. You can carry milk (get the kind that sits on the shelf) and then you do have to tell TSA that you use it to treat lows. When we were first diagnosed, milk was the only thing I could do to bring up lows. Now that Will is 4.5 years old, we use glucose tabs. The TSA may give you crap about this, but if you have those sealed milks, they are actually pretty good about it. If they make you throw it out, just purchase a small carton of milk before you get on the plane and ask the flight attendant to put it in the fridge for you. Explain why and they are really cool about it.