Been to Paris with DS when he was 8yrs (+DD 5 at the time),
He loved the Eiffel tower, he liked going up in the lift because it was a bit scary. He hasn't been on the river tour but when I went with DH he fell asleep, I didn't rate it much either as you are quite low in the river so no veiw isn't that great, bus tour would give a better view perhaps. We got the Visite Pass for the Metro and jumped on and off that all day. It can be bought at most Metro stations, just point to the day and zone combination that you want on display at the kiosk if your french is a bit rusty.
He also recomends the big science centre Cite , it has the submarine The Argonaute to explore, The Geode imax cinema, a Plantatrium, a small aquarium plus loads more, it also has Cite des Enfant which has two areas for over and under 5's full of interactive exhibits that you and your children go round together. There are different tickets that include all or some of these different areas.
He wanted to pass on his eating place recomendations which are Flunch (fast food French style, well more self service as they don't do burgers) and Hippopotumas ( another chain that seems to specialise in steaks and fries).
Where are the catacombes??? We tried to find them, is it the same as the sewers?