It can depend on the size of the dog and the quality as well as quantity of the chocolate too. Our Labrador (who else) once ate just about an entire dark chocolate children's birthday cake . The only ill effects were that she sicked it up almost immediately after and had the runs for 3 days, plus she looked very sorry for herself.
The last greyhound demolished an entire (small) box of chocolates, including wrappings destined as a Christmas present for the cleaner. All I can say is I must have bought cheap chocs as she was also none the worse.
However, if we are talking posh dark 70%+ choc, then maybe the vet would be a good idea. Easter eggscare usually very thin and generally milk chocs so the dog may be fine. Hope so!