Which part of the coast are you looking at?
If you are looking at the West Coast, I highly recommend Tofino on Vancouver Island. Any resort on Chesterman Beach is good. This year, we spent March break there. The weather was nice and mild, no rain. There are miles and miles of beautiful sandy beaches, rocks to climb for my DD (age 6), waves to chase for my other DD (age 3), starfish and sanddollars to find, and beautiful sunsets to watch. I could go on...just let me know if you want more info...