Ideas for where to take the kids for a daytrip to the beach?

Avoid, the North East coast, Ostende, De Haan, Blankenberge, Knokke Heist. It's crowed and very expensive.

The "quieter" beaches are South West, either side of the French border. Koksijde, Saint-Idesbald, De Panne on the Belgian side, and Bray Dunes or Leffrinckoucke on the French side.

Frankly however, with this weather, it's all going to be busy everywhere, but the south western beaches are significantly less crowded.

Go early, i.e. leave 07.00 / 07.30 am to get there at or before 09.00 am. That way you'll find parking, and a decent place on the beach itself. You can also have lunch early before the crowds. You'll also be able to leave by 15.00 / 16.00 having had a full day on the beach, rather than arriving after lunch, stressing to find somewhere to park, getting upset and shouting at your kids to find somewhere on the beach amongst the crowds, and then leaving at 18.00 / 19.00 in the evening, and getting stuck in 20km tail backs on the mortorway.

If you go to the "broder" beaches, you also have an alternative route back home via Dunkirk / Lille / Brussels.

typo - should have said "border" beaches

Try De Gavers in East Flanders, Belgium. They have a beach, canoes etc.
https://oost-vlaanderen.be/ontspannen/recreatiedomeinen/de-gavers.html

I usually go to Vrouwenpolder (a very long and wide strip of sand with a lake the other side of the dam, good parking and a cafe) or Kamperland in Zeeland. Much less crowded than the beaches in Belgium and about the same distance from Brussels, usually without the crazy traffic jams. Also check out the lovely town of Veere very near Vrouwenpolde.r