Do an early evening/twilight boat trip up the Seine (some good ones leave from the point of the Ile de la Cité) - they are great and usually timed to pass the Eiffel Tower on the hour to see the lights flash. The boat tours usually go right in front of the tower so you get a very good view.
You can get walk up tickets for the tower - we were actually surprised at how small the queues were if you go quite early in the day.
The Ile St Louis for Berthillon ice cream at the Berthillon shop is not to be missed! Then cross over the Seine for a walk round the Marais — great for window shopping and nice bistrots/cafes and the Place des Vosges is lovely. When we were there there was a nice pop up cafe in the lovely courtyard of the Musee Carnavalet.
We went last year with my 12 y o who loved it, (but she is arty and does like museums — yours may not). Saw the Louvre greatest hits & the IM Pei pyramid, the Pompidou Centre, the Musee d’Orsay, the Sainte-Chapelle. Highlights the 12 y o liked most: the Eiffel Tower, the boat trip, going on a double-decker RER train (between the tour Eiffel and the Musee d’Orsay); the ice cream; shopping at the museum shops; the Tuileries; the Pompidou; steak haché.
One funny thing about Paris (if you haven’t been for a while), is that currently, English style, food etc. are very much in fashion! So lots of adverts with English in them; lots of the city was done out with English-style grass and garden planting for the Olympics; and trying to find authentic French food is nigh on impossible because everywhere seems to be serving “fashionable” English food, Heston Blumenthal style. Think Aberdeen Angus steak with earl grey foam and “English” chips on all the upscale bistro menus! French food seems to have been temporarily replaced by burrata and heritage tomatoes, and stuff you’d expect to see at a gastropub here. And weirdly vegan salad bars are suddenly the dernier cri. So you may find it easier to get food that a 12 y o will eat than you’d think.
Enjoy! Let us know how you get on!