I am so sorry to hear about what happened upon your arrival into Paris and that you did not get to stay and see the city.
I have taken the very route you did into the city but it IS a very difficult thing to do, as you mentioned, riding the RER line B, then transfer to the Metro and exit at Chatelet - I had to do that my first time in Paris but I was alone. The trains were packed and I think unfortunately, this is when things tend to happen because things don't get noticed in large crowds and getaways can be fast and easy. That someone actually TOOK your child is terrifying, I can't imagine what was going through your mind except saving him and you did, thank goodness.
There's all sorts of way to "armchair" quarterback this mishap but that does you no good now. I wish you could have enjoyed your extended holiday. Indeed, in big cities, including Paris and all around the world, these things do happen and there simply aren't enough resources for the police to investigate. The fact that this was an attempt at a child abduction makes me wonder why the police were not more concerned in this instance except, and maybe they did not explain this to you, this has happened before and it is more an attempt to take the backpack than it was your child, although, what anyone could consider taking from a child's backpack worth taking...we are living in strange times. Europe is in crisis and although maybe France isn't yet in the news except for the recent elections, the rest of Europe is struggling and France has been struggling with immigration problems for years. My home city is no different.
Poor economic conditions, big city crime - tourists are targeted - is a sad reality. I have been to Australia and I know things are quite different there, especially out west in Perth. I know that Sydney and other eastern cities have crime, but probably NOTHING on the scale of many other cities around the world.
I've seen crimes committed against tourists on the metro; it's sad but these criminals are "pros" and almost before anyone realizes anything has happened to them and it is quite unsettling just to see happen, let alone be victimized.
We like to think our holidays are going to be "idyllic" and often they are but sometimes bad things happen; the severity of incidents can vary and maybe not be as extreme as what you went through or sometimes, can be even worse. For a neighbor or mine, it was as worse as it could ever get.
You did what you felt you had to do by leaving but I hope sometime in the future, you'll get another chance to try and do that trip - it was a big undertaking on your part, going with your three young children with you. It's something many wouldn't even try. I am so sorry you had such an awful experience but am glad you are all OK.
Let us know if you get any reply from your letter. Consulates can often be helpful when abroad in a city like Paris, but there are limits to what they can do; I don't know if they could have done more to help but maybe....writing the letter at least helps them to know crime statistics and critical areas of concern that need better monitoring/manpower so you did the right thing and hopefully writing was a bit cathartic.
Don't give up on travel....maybe wait a while but give it another go. We help many families plan their trips and share advise in advance about what to be on the lookout for, options etc. I'm hoping someday you will all have a lovely European holiday. Meanwhile, take care - feel better and when you're up to it, maybe come back here and tell us about all the wonderful things about Perth......it's on my list for when I get back to Oz, and I'm not the only one who's thinking about Perth either