Here's another vote for taking a taxi.
There have been times when those 15 additional euros for the taxi would have been prohibitive for me, so I can appreciate it if that's where you are.
But aside from that, the taxi wins hands down. Pickpockets are always a concern, but also consider that you might be arriving jet-lagged (you don't say where you're coming from), and will have to negotiate the transit system with all your luggage. The metro and RER are relatively easy to navigate, but if it's your first time there will be a learning curve and some of the train changes can involve quite a bit of walking.
For a family of 4, 15 euros can easily get gobbled up in an expensive city like Paris. I'm not sure you could even all get an ice cream cone for that amount. So you could look at it as missing a small treat in favor of a much smoother, much more hassle-free arrival. You'll be glad if you take the taxi.