5 days itenary in Paris

Very recently someone posted on this forum that you can no longer buy a ticket on Level 2 for the elevator to the top of the tower, so I believe the answer to Question #2 is "No".

I suspect restaurant reservations at the Eiffel Tower are very much required, but it's massively out of my price range, so I've never delved into it. You could start the reservation process and see if there's a calendar showing available days and times. That would give you some idea about the situation.

Here's a link to the 58 Tour Eiffel Restaurant on the first level, the less expensive one.

And this is a link to the Jules Verne Restaurant. I have no idea whether you might run into a rule about young children there.

One full day at Disneyland Paris would be enough for the children, it's not as big as the Orlando version. Eating at the Eiffel Tower restaurant is overrated but if you want the experience of eating and seeing the city than it might be worth it. There is a wonderful Childrens Science Museum just across the River Seine from the Eiffel tower that the kids would really love, mine did. Go to Luxembourg Park and the kids can rent sailboats and float them in the large pond. (may be to early for this) Be sure to take the kids out at night and watch the Eiffel tower light up on the hour every hour. I would take them on a cruise of the River Seine maybe even a dinner cruise. Hope you have a great time!

No reason at all to rent a car, it would just be a problem for you the whole time. The RER "A4" train will take you to Disneyland, and the Metro and buses will take you anywhere you want to go in Paris itself.

Maybe reduce your days at Disneyland to one or two days, so then you could have another day or two in Paris. Your kids will enjoy Paris too. There are some great parks where they can run and play (perhaps the Luxembourg Garden or Tuilleries).

I know that it can be tough to bring kids to museums, but maybe do short visits to a museum. I find that my kids like me to take a picture of them mimicking the pose from a sculpture or painting as they are standing next to it. It is a nice way to involve them in seeing the art and for me it means that I can go to the museum and see the great art and they can have a little fun there too.
I find that my kids like the "hop on hop off" buses, for some reason. It's a nice way to see the city and the kids generally enjoy sitting up top of the double decker buses. ;) We did the hop on hop off bus around Dublin last year and they could have rode that bus all day.
I'm going to Paris this summer with my children (10 and 8) and we'll be spending one day in Disneyland and 2 1/2 days in Paris. I looked up the train times online so I had a general idea of when to catch the train from Paris to Disneyland. They seem to run about every half hour.
Have fun on your trip!

We visited the Eiffel Tower last May and ate the restaurant Le 58 Tour Eiffel - in my opinion it was not good and it was expensive- I would suggest you eat elsewhere - you can walk and find many good cafes in the surrounding area.