While the first post on Gallic sanitation wasn’t really geared to the very young, kids of all ages like visiting the Égouts de Paris (Paris sewers) and yours would doubtless enjoy doing so if you’re ever in the area once we can all travel again. The first sewage system in the City of Lights was just a series of thirteenth-century troughs running along the middle of every road, but the first underground system was started beneath the Rue Montmartre in 1370 and expanded and upgraded down through the centuries, with the end result being the system started by Napoleon Bonaparte in 1850. Even back then it was a popular, if somewhat unusual tourist attraction, an attraction amusingly promoted in the 1962 Disney film, Bon Voyage. Which your kids would also enjoy, but take note: the sewers aren’t open to the public on Thursdays & Fridays.
https://www.mtholyoke.edu/courses/rschwart/hist255-s01/mapping-paris/Paris_Sewers_Page.html
Time Travelling With Kids 