Wow! I love your enthusiasm! I'll try to help...
Only one day? Let me think out loud here about how much time to allow for those sites. Eiffel tower - plan on 2 hours for lines, photos, finding your way, having fun. Notre Dame - maybe an hour if you don't want to walk up the stairs for the view. I've been to the church recently, but not up stairs for the view. I think it is just a walk up, no elevator!
Louvre - wow. Amount of time depends on your objectives. Does your 21 yr old like paintings? I'm pretty sure strollers are not allowed in the museum, but actually I don't know! I will glance at the website.
One strategy would be to just go there and each person choose one area to visit. If you want to follow everyone to the room with the Mona Lisa you could...I like the large size French 19th century paintings - Delacroix, Gerricault. They show scenes from French history and are very impressive.
I love the gift shops in the Louvre; they have nice souvenirs, some small items, decks of cards (Mona Lisa of course), pens, pencils, small mirrors, letter openers, fans (they have gift items in a Marie Antoinette type collection that are from Versailles, also in this location).
Right across from the Louvre of course is the Tuileries gardens. If your 5 year old needs a run and play break, this is the spot. Lots of chairs around the pond; you can rent boats in summer. Across the street (Rivoli) you could go to the tour guide famous Angela's café (or is it Angelina? I can check) for the super rich hot chocolate.
So what season/month will you be there?
I wouldn't worry one bit about being there after dark, especially if you will be back on the train by 9pm. Just usual big city style precautions apply - carry your purse on the side away from the street, don't put it on the floor in the café, have your subway tickets out and ready before you get into the station, etc.
Stroller: would you use one for your 5 yr old in Disney? Running around Paris is as much walking as doing Disney all day! I would say skip the stroller and just take sit down breaks in parks or cafés, in the church. But I haven't travelled with a little one in a while! If you aren't sure, I'd say get the cheapest umbrella stroller and bring it from the States, maybe with the idea of just abandoning it if it's too much trouble while in Paris.
I'm happy to share more ideas/opinions. These are just my ideas - I hope others will join the conversation!