At least seven months prior to taking our 16 year old nephew to Paris in 2015, my husband gave him a slim guide book (and some links to various websites), along with a request to follow up a month or two later to discuss his thoughts. Above all else, getting our nephew's input (he specifically requested smaller, less crowded sights & museums) was instrumental in making our trip a memorable and enjoyable experience.
In the end, his favorite museums were the Musée de Cluny (due to reopen in early 2022) and the Musée Carnavalet (due to reopen very soon). Also known as "the History of Paris Museum", my nephew absolutely loved the Carnavalet. At one point, we split up for a short while and when we regrouped, he excitedly told me that he saw THE actual painting that was in his history book (can't remember which one) ... he was blown away to see the original painting IN PERSON, that was reproduced in his history book at school. There were loads of interesting objects & exhibits, and because the museum is housed in former private mansion in the Marais, just walking through various rooms (and the beautiful gardens) was memorable. Moreover, all of the exhibits are translated into English, and according to the museum website, "10% of the works displayed have been installed at a child’s height". For what it's worth, on each of my four visits, I've encountered small groups of French school children who were as enthusiastic as my nephew. Here's a link to the museum website: https://www.carnavalet.paris.fr/musee-carnavalet
Afterwards, you can visit the nearby Place des Vosges - one of my favorite small parks in Paris (technically, it's the oldest planned square in Paris, built by Henri IV in 1612).
On the food front, the Marais district is home to some of the best falafel in the world. These inexpensive and delicious Middle-Eastern sandwiches are composed of deep-fried round chickpea fritters, grated or chopped vegetables like cabbage, carrots and/or cucumbers, and sesame tahini and/or hummus. Several competing restaurants (with take-away options) are located along the Rue des Rosiers. As one who eats very little meat, I welcome the occasional falafel as a break from the typically rich cuisine normally found in Paris, and eating your take-away sandwich in the park makes for a lovely picnic.
One other note regarding the Marais: as it was the old Jewish quarter, I think it's important to pay attention to, and pause momentarily at one of the moving plaques placed outside schools, which pay sad tribute to the more than 6,000 Parisian children sent to death camps during WWII - the vast majority of whom died. One such plaque is posted near the entrance to the school located on Rue des Hospitalières-Saint-Gervais, a pedestrian street just off of Rue des Rosiers.
Here a link to a website with more info about these plaques: http://www.alamemoireparis.com/overview.html
Also, do keep in mind that Paris gets brutally hot & humid in the summer. We were there with our nephew in June, and didn't cover nearly as much ground compared to when my husband and I normally visit in early spring. If you aren't used to that sort of weather, please understand that it can be exhausting - especially for kids. Obviously, be sure to drink plenty of water, but also beat the heat by trying the various ice cream shops - glace in French - to find your favorite flavor.
One last note: don't underestimate the sheer amount of walking one does in Paris; even when using the Metro, you still clock a fair number of miles & steps. That, combined with heat & humidity, can cause some serious chaffing. Thank goodness for an anti-chaffing skin protectant product called "Glide" ... it literally saved our vacation.
Here's a link for it at REI: https://www.rei.com/product/745879/bodyglide-anti-chafe-formula-skin-protectant-15-oz
Hope this helps.
