I could write a very long message on Paris. My wife’s family is from there so we’ve been numerous times. I’ll try and put something together including a couple of favourite hidden gems.
RER B - the main line from the city to CDG is undergoing a major upgrade ahead of the Olympics. I believe they are generally trying to keep things running fairly regularly for tourists but when I was there in October there was some shuttle bus service before getting on the train. I’d check the airport site for the latest updates.
https://www.parisaeroport.fr/en/pass...ccess-airports
There were a pile of very helpful guides at the airport and at the shuttle bus stations which made it fairly easy going back to the airport. I was alone on my arrival and it seemed the middle of the RER line was closed on the Sunday I arrived so the train service didn’t sound as straightforward and ended up with car service thanks to my FIL.
With kids you may want to like at a service as it would be one less thing to worry about when you’ll likely to be overly tired.
As for precautions, it’s like any large city. The guys selling the trinkets around the tourist spots aren’t a worry. The normal situational awareness you have as a tourist in any city is fine for most areas. Blend in as best you can. I’d be a bit more cautious around Montmartre as that area seems to have more of the bracelet crowd. There is also some weird scam where the person drops a ring and says it’s good and tries to sell it to you. Pigalle and the area around Moulin Rouge is a bit red light districty. Like anywhere, keep your bags closed, keep them in front of you if possible in the really tourist areas and on the metro.
Do use the metro. It isn’t costly and there are stops everywhere. There are a pile of different apps you can download which have the entire metro plan and will tell you how to get from point to point.
It’s a great city with tons to see, enjoy it.