The only problem with May in France is that it's peppered with bank holidays ... which are observed on the actual day not passed forward onto the most convenient Monday as in the UK.
So, for future planning, avoid
MAY 1 Workers' day (Fête du Travail)
MAY 8 Victory in Europe Day
Ascension Day - a THURSDAY 40 days after Easter .......... and many French take the next day - the Friday - off to make a long weekend (often starting on the Wednesday as French children do not go to school on Wednesdays, up to the equivalent of Year 9),
and depending on when Ascension falls, BEWARE of Whit Sunday (Pentecôte/Pfingsten) and Whit Monday which are public holidays in both France and Germany.
For alternative routes on packed stretches of motorway, the Bison Fute (crafty bison) website is a bit help and also shows places en route where you can stop and the children enjoy entertainment (animation).
OK so this might be considered
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for Germany, but in my area, the holidays in both countries influence most things.
And Europapark is in Rust (Germany) about 40 mins from the French border near where I live. I second that it is a super theme park and has at two hotels in the perimeter - the Colisseum and the Spanish hotel (?Andaluz?) and there are also many guest houses very close by which are sensibly priced and very clean. GO FOR IT!!!