I recall reading a series of French books in the 1980's about a group of kids who had some time travel adventures (maybe among other types of adventures). The books might have been pre-1980's
In one of the stories, the kids were travelling to the future, but ended up in a medieval time. Eventually, it was revealed that they were indeed in the future, but there had been a nuclear incident in the area and society had organized itself in a medieval fashion. The area was in France, maybe Bretagne and generally in a forest setting.
Another book was about how one of the kids had Viking ancestors, and he was navigating a Viking ship as if by instinct, he was remembering how to navigate by stars.
Another plot, perhaps in another book in the series: their time travel had accidentally brought in some dinosaur into medieval times, and they had to collect the ingredients for gunpowder (salpetre?), and then blew up what the medieval folks believed to be a dragon, maybe into a cave.
Pretty sure it was not "Les 6 compagnons" or "Le club des cinq"