London/Paris

Could you fly home Sunday? With the time difference you get back home the same day. This would make your plan less tight, with 6 full days instead of 5, adding the extra day to Paris.
If it's not possible... Maybe your daughter has a preference between Paris and London? In which case, pick one.

It’s doable but if another day was added on either side of arrival and departure dates would help.

I Understand the short flight from NYC is a benefit and curtails some jet lag but with so much to see you do need time to allow mind and body to adjust.

Not to mention who knows what your arrival Heathrow experience will be like and depending on your accommodation choice you’ll need to factor in time to get there, unpack, etc.

Pre book must sees in both cities. I’m a big advocate of a first timer enjoying theatre in London. Have her review what’s scheduled to be on during your dates and if it’s a popular play consider pre Booking directly from the Theatre’s website.

Or you can always take time to see what’s available via the 1/2 price ticket booth. Not the major shows unless like Phantom they’ve been playing for years. Now available to review on line but you still have to go in person to pick them up.
https://officiallondontheatre.com/tkts/

If you can’t add a day or two. Pack lightly and hit the ground running. Have a daily plan. Helps if you double check opening and closing times of the sites, especially on Sundays.

She’s going to love this trip. Bravo

What a wonderful idea!! It is a rush rush trip, but she will get a taste of both cities. Just make sure you plan ahead to make the most out of your time.
It would be ideal if you could add a few days, but I understand that's not possible. Anyway, you can rest on the plane and on the train from Paris to London.

Honestly? With just 5 full days, I'd probably pick one or the other. Since you've been before you know there is plenty to keep you busy for 5 days in either location.

IF your daughter has her heart set on seeing highlights of both, then by all means do it. You've got knowledge of both cities so you can edit down to what sights might be most important/fun for her.

If you are returning to work on Monday, I'd fly home on Saturday as well.

She’s 19 — she will thrive on the pace of your trip while absorbing all she experiences! Have a great time!

I think returning on Saturday is a smart idea given she will either have classes or work on Monday and it’s better to have Sunday to handle the jet lag. You too! 😊

The young typically favor quantity over quality (they can say, I've been to Paris AND London, instead of just one, haha!). I'd shoot for both, and whet her appetite for when she can return independently in the future.