Hard to point to historical fiction that is suitable for kids, because so much of it is explicitly written for adults. A good history book for kids (with illustrations) might very well be the thing to go for; e.g., like the cartoon history of the universe, mentioned above.
If you have the opportunity, I’d visit one of the museums that focus on royalty in France, e.g., Versailles, Fontainebleau, a medieval castle… that sort of stuff. Both because it’s a fun experience - you’ll get to see how the royalty lived in the old days, but also with a little luck you can probably find some suitable and interesting history books in the museum shops related to the historical period of the castle/chateaux, etc - often specifically written for kids.
In fiction, the only thing that I can think of (other than Dumas) which is not for adults, is Sir Arthur Conan Doyle’s duo of historical novels: The White Company and Sir Nigel (yes - he wrote other stuff than Sherlock Holmes - and allegedly preferred these works to his famous detective). Both are novels in the “good old” Victorian tradition, with lots of jousts, chivalric deeds, and battles - but well researched, and the characters are a delight. The writing style may be an obstacle (Victorian english writing, with the dialogue written to sound appropriately “medieval”) - if you/he can get past the writing, though, they’re rollicking good adventures for the boyish and boyish at heart.
> It is in the heart of the great Cistercian monastery that this chronicle of old days must take its start, as we trace the feud betwixt the monks and the house of Loring, with those events to which it gave birth, ending with the coming of Chandos, the strange spear-running of Tilford Bridge and the deeds with which Nigel won fame in the wars. Elsewhere, in the chronicle of the White Company, it has been set forth what manner of man was Nigel Loring. Those who love him may read herein those things which went to his making. Let us go back together and gaze upon this green stage of England, the scenery, hill, plain and river even as now, the actors in much our very selves, in much also so changed in thought and act that they might be dwellers in another world to ours.
