Best beach resort between Amsterdam and Paris

FYI:
https://www.we-heart.com/2022/10/12/luxury-beach-resorts-in-the-netherlands/

There is forum member based in the Netherlands that might chime in

In France, the one place that comes to mind is Le Touquet, but you'd need a car, ideally, and the beachfront is just ugly buildings. The charm lies in the streets behind the beachfront. There are some good hotels with spas, pools, etc.
Anywhere between the Somme and the Seine rivers is pebbly, so probably a no-go for you.
South/west of the Seine (i.e. Honfleur and points west), beaches are sandy again. Deauville and Cabourg are very pleasant resort towns; Deauville can be very fancy.

Now, this being said... Personally I'd save some time off and enjoy the beach closer to home in California or Baja California.

Belgium has lovely some sandy beaches between Knokke-Heist and Dunkirk, but there’s a reason people focus on the beaches further south, in France - the weather further north is unreliable and the water is freezing, especially at this time of the year.

It's unlikely that you'll get bathing suit weather and a luxury beach experience anywhere along the coast you specify. Windy and cool mostly.
Sure, you could get lucky along the North Sea, but I'd go farther south to the Biarritz area - more than 100 years as a warm weather beach vacation hot spot.

As others have said you’re too far North for beach pampering. There are some luxury hotels on the French coast at Deauville and surrounds but the weather would not be guaranteed so it depends what you’re after really.

In general the best time for going to the beach in the Netherlands is during high season in July, August and September can still be good too. Water temperature will be around 20 °C. Most of the time there is some wind, sometimes carrying sand and the reason you often see windshields here.

Popular places here are Zandvoort and Scheveningen, so busy during high season, the latter to my opinion the most attractive of the two. And easy to get to The Hague and other places like Delft and Leiden. In general I don’t fancy the beach resorts in the Netherlands however I would go to for instance tiny but cute Domburg in Zeeland or the island of Texel north of Amsterdam. Both are busy during high season too. Domburg is an outlier but with lovely places nearby like Veere, Middelburg and having a car a daytrip to Bruges doable it's worth to consider to my opinion.

Knokke in Belgium is posh, the best part is along the Kustlaan / Het Zoute. De Haan is lovely with it’s Belle Epoque architecture, from both places easy to get to Bruges. However the Belgian coast in general with it's monotonous omnipresent appartment blocks is not directly appealing.

About France I agree with balso. Cabourg, Houlgate, Deauville and Trouville-sur-Mer are very charming and within easy reach from Paris. Like to add La Baule-Escoublac west of Nantes, it has a bit of a micro-climate and a very pleasant climate and vibe. Nantes to reach by train in two hours from Paris.

All in all I would look for a place in France, better climate and more attractive places.

Not a great area for beaches. There is far more great stuff to do that the beach.
In fact, having lived in Europe for four years, I never found any great beaches that were as nice as what you get in Florida or California.

I would suggest Le Tourquet also, was there once in 1999....a good visit.

The mild climate in the western part of Europe is because of the warm Gulf Stream. And explains the climate difference between Madrid and New York who are at the same latitude too. The extreme cold winters you have in New York are almost completely absent in Madrid or anywhere else (maybe with the exception of Norway) along the Atlantic coast here in Europe. Nevertheless with summer temperatures over 30 °C in the Netherlands and Belgium many are happy going to the beach to cool off here.

For some perspective: Seattle and Paris are at similar latitudes and Amsterdam is close to the midpoint between Vancouver BC and Ketchikan in latitude.

Completely irrelevant. As already pointed out, the Gulf Stream plays a major role in the climate of Europe so despite cities being at similar lattitudes their climates are typically incomparable.

As for answering the OP's question, perhaps somewhere in Brittany might appeal. Some very nice beaches coupled with attractive seaside resorts and not too far from Paris.

Thank each of you so much for your invaluable depth of knowledge. And Balso, I believe you are right! California beaches are probably the best bet when we return home. A leisurely journey to Paris will certainly provide all we can possibly hope for.