eiffel tower, palace of versailles, louvre- if buy tickets in advance,skip the line? and other ?s

If you have timed tickets to the Eiffel Tower (bought 90 days out) you are admitted to the reserved elevator at the time of your reservation. If you have tickets to the top then you will stand in an elevator line at level two to get to the top and that can take awhile. If you don't have tickets to the top then you can buy them on level 2 which entails another line. Most people young enough to have kids the age of yours will have no trouble walking up the Tower.

If you have tickets to the Palace of Versailles you do not skip the worst line which is the security line; everyone is in the same one and so it is important to get there at least half an hour before the palace opens. I frankly would not do Versailles with kids this age. It is beyond crowded and they will probably remember it as a misery where they couldn't see anything. Big glossy gilt rooms are not generally that fascinating to kids anyway. The garden is free except on fountain days when it requires an additional ticket. I think it is worth visiting on a fountain day and there is something to be said for going out for the Hameau, the gardens, perhaps the Trianon but with limited time, I would skip this one:
https://janettravels.wordpress.com/2016/02/15/fountains-of-versailles-a-little-chaos/
If you have a museum pass or tickets to the Louvre, you can use a different shorter security line at the Pyramid; kids just walk in with you for free. The same is true at the Orsay where you use a different door.

If your kids are good bike riders and a bike tour would be fun for everyone, then Fat Tire has a good reputation.

I was there 3 weeks ago, and when I was walking down from the 2nd level to the 1st on the Eiffel tower I saw a good number of reasonably fit people climbing up, you can always rest in between, but with 2 kids the elevator is probably your best bet.

If you have a reserved entry time for the Eiffel you won't really wait in a line--everyone goes through security, then they allow you in based on your ticket time. I presented myself at security at 10:20, 5 minutes to get through and then on the elevator at 10:30. Decided I felt ok and purchased my supplemental to the summit--again, no line, just got in the elevator.

If you have booked your Eiffel Tower tickets in advance, you can enter the grounds ( presently there is a fence all around the tower), through a separate entrance ( next to the general entrance)
with a green sign that reads; "visitors with reservations".
Once inside the fence, you can go through a different line (look for the same green sign), but only at the time specified on your ticket.
You may have to wait in line, depending how many other people had the same time as you (sometimes a tour group of 20-30 people may enter before you & you may have to wait longer).
You then wait to go through a second security check point, similar to the ones found at airports where all bags/purses are placed in conveyer belt & are scanned via X-ray machine.
Afterwards, your ticket will be scanned by a staff member, then you wait for the elevator. They stuff you in the elevators like sardines. Once you get to the second level, you wait in another line to go to the summit (if you have purchased such a ticket)
There are also lines to catch the elevator(s) going down.

The views at the second & summit levels are spectacular!

If you buy a Paris Museum Pass, you can enter any of the monuments covered with the pass, via a separate line, but you must first go through a security check point.
Enjoy your Paris visit!

To elaborate on the last note -- there are separate security lines for pass holders at many sites including the Orsay, Louvre and Orangerie so it is not the case that you need to stand in the long security line with everyone who doesn't have a ticket. Having a ticket or a pass greatly reduces line time. At Versailles, there is just one security line.

Ok..so if I had a museum pass then I enter a different security line at the Louvre, which might be shorter than the security line I enter if I only merely prepurchased advanced tickets?

I haven't bought tickets for anything yet, the only available times for 2nd floor Tour Eiffel are at 11pm! :( so my only options are to walk up the stairs or buy via a tour company for double the price just to get to the 2nd floor or summit ..

No those with a museum pass OR a ticket can use the pass line which is currently at the Pyramid. Sometimes it is the Richelieu entrance. We generally use the Carousel entrance and the line has not been a big deal there but it is better known now so it may not be as easy as in the past. No special line for passholders there.

Same at the Orsay; museum pass or pre ticketed people use a different door/line than those waiting to buy tickets.

I was at Versailles a couple of weeks ago and the security line was very quick but the line to get in was ridiculously long. And yes, everyone has to stand in that line.

You know your kids best, but I think you should reconsider taking them to Versailles. I saw a lot of very bored and unhappy kids there. I felt sorry for them. They can't appreciate it and it's torture for them in such crowds.

Thanks all, decided to skip Versailles ... Is palais royal in Paris worth a visit? I read that was home to the king prior to Versailles?

Now debating if the museum with it for us versus buying indiv tickets ahead, I've looked at the pass website, seems like a good deal, any kids prices though? Didn't see them on there

The Louvre is the home of kings in Paris and so if you want to see a chateau you can kill two birds as it were. There are also several chateaux easily reached by train that are not crowded including Vaux le Vicomte and Fontainebleau. Here is Vaux le Vicomte; its designers did Versailles.
https://janettravels.wordpress.com/2010/09/20/vaux-le-vicomte-our-last-chateau/
Within the Louvre are the 'Napoleon apartments' which were the home of Napoleon III. So the museum as a whole was the residence of the kings of France and you can see the lavish rooms of a later emperor.