Using EF 6.1 for updating objects and their children
You don't have to use a using statement. You can use an IoC container such as Autofac and let it dispose the PollContext for you.
I think you probably should update the children collection in memory first and call context.SaveChange() only once at the end, so the whole update operations are wrapped with in a unit of a work.
I don't think try/catch is needed here as you only re-throw the exception, which could be handled on a higher level.
There's lots of repeating of:

context.Entry(poll).State = poll.PollId == Guid.Empty ? EntityState.Added : EntityState.Modified;
context.Polls.AddOrUpdate(poll);


You should probably create a generic function for it.