The Metro was really easy to use, although lots of the stations only had stepped access.
Well done for trying this.
We did it last summer, with a 5 and 9 year old. Stayed in an apartment and walked or took the Metro everywhere. We easily walked 5-10km each day, so do think about what you actually want to do there.
I made up cheese/bread/apples/another fruit or veg/1 treat picnic lunches for everyday, which keeps costs down.
We found a brilliant book 'Mission Paris' which was a clue hunting game for children, around the sites.
I recommend taking the No. 1 line to Vincennes (last stop, and still in the Metro ticket). It was the French Royal Palace, before Versailles, and has a brilliant Medieval castle, with an English, kids appropriate audio guide.
When are you going? Your DC are also old enough to be able to swim in the Seine, if that floats your boat?
Lastly, we were also surprised by the cost and ease of using the Eurostar. You can get a Metro to the Gard de Nord, which was easier than getting to either airport, and the cost was about 1/3 that of the plane fares.