We're heading to Disney for a couple of days and will have one day in Paris itself. We did the same in August and as it was so hot didn't do a huge amount. We did a boat trip and only managed to get off for The Louvre (surprisingly popular with the children and an escape from the heat) and the Eiffel Tower (didn't go up it).
We'd like to do one art gallery (thinking Pompidou as DH is architecture buff and the quirky art might be fun plus view from the top of the building), Notre Dame and Montmartre. Maybe squeeze in a quick stop at the Arc De Triomphe. Not sure if this will be too much for a nearly 6 year old and 8 year old.