Hello,
Would someone know where on Sri Lanka southcoast we would find a beach that meets all of our criteria :
- you can swim safely
- beautiful place : white sands, clear blue water and coconut trees
- not a party place but also not an isolated area, we prefer when there are some restaurants, shops ...
- good snorkeling (if possible)
We are 2 aduts and 3 children 9 to 13 years old, and would stay 3-4 days in february.
Thanks

I don't think you're going to get everything on your list in one beach, you're going to have to prioritise. Lots of beaches have clear water, swaying palms and restaurants and shops but white sand and snorkelling are harder to find.
White sand is difficult, there are the odd coves here and there with white sand but no infrastructure. The compromise would be Mirissa which also has lots of restaurants, nice swimming and whale watching but no snorkelling to speak of.
Snorkelling is best in Hikkaduwa, at the far northern end where you can easily swim with turtles and see the usual tropical fish. The sand is not white but swimming is safe, loads of restaurants and shops in the area.
Horses for courses.

OK, I see the difficulty ... and my guide says the swimming in Mirissa is dangerous ...
Would you see other options if I forget about the snorkeling , and say yellow instead of white sands ?
Thanks Checkingirl
Never heard that swimming is dangerous in Mirissa. Everyone was swimming and the bay at the southern end by the rock is particularily sheltered.
Unawatuna IMO was lovely when I first visited some 20 years ago but had turned into a concrete hell-hole a couple of years ago with virtually no beach left. it's the only place in SriLanka that I have no desire to return to.
Tangalle is OK. Long stretches of beach rather than coves and the sand is coarse. No snorkelling to speak of but kayaking on the lakes is nice. Good beach-front restaurants.

Here is something I and others wrote about S coast beaches https://www.lonelyplanet.com/thorntree/forums/asia-indian-subcontinent/sri-lanka/beaches-on-south-coast?page=1
Have a look.
As you can see, I like Tangalle, but thought the sea a bit rough for safe kiddy swimming. They have built mini breakwaters, but there is fairly rough surf if you venture out of these areas.