Belgian coast

Look up the tourism offices of De Haan or Koksijde on the internet. Both are easily accessible and very pleasant resorts. Train from Brussels to Ostend and tram to either of them.

Getting to any of the resorts on the coast is easy from wherever you live in Belgium. There are trains at least every hour to a range of stations and the Coast Tram runs from De Panne station - actually in Adinkerke - at the French end to Knokke at the Dutch end and stops at all the other railway stations en route.

I always find that Blankenberg is the easiest place to get too and stay. Direct trains from Brussels Midi or Central station very regular and takes about 1hr 20 mins. From Blankenberg there are endless places to visit by tram along the front as mentioned in other posts above for info see: http://www.blankenberge.be/Toerisme/English/Home/page.aspx/2058 Cannot advise for hotels as is your choice of costs and location but there are loads within easy walking distance of station.

With the greatest of respect Meha, the OP is looking for somewhere quiet.
Blankenberge is the Belgian equivalent of Blackpool, Brighton and Coney Island all rolled into one.

Hotels tend to be busy in August and it can be hard to find rooms. But we are coming near the end of the holiday season. If you can wait until end August and kids are back in school, then everywhere is quieter and cheaper

Ostende has a long beach.

I went once to Wimereux in France for a weekend and it was wonderful. Its quite a long way (change of train) but you can go easily by train and foot and emerge beside the beach. Wimereux is off the tourist trail entirely and has a nice beach. its just a village. The train stop is Wimel Wimereux I think.