static void Main(string[] args)
{

    var client = new MongoClient();
    var server = client.GetServer();
    var db = server.GetDatabase("test");
    var collection = db.GetCollection("ParentChildren");
    BsonClassMap.RegisterClassMap<Parent>();
    BsonClassMap.RegisterClassMap<Child>();

    var parents = collection.FindAllAs<Parent>();

            
    foreach (var parent in parents)
    {
        Console.Out.WriteLine(parent.Name);

        foreach (var keyValuePair in parent.StringToChildren)
        {
            var child = keyValuePair.Value;
            Console.Out.WriteLine("Key: "+ keyValuePair.Key +" Name: " + child.Name);
        }
    }

    Console.Out.WriteLine("Press Enter to exit");
    Console.ReadLine();
}

public interface IParent
{
    [BsonId]
    BsonObjectId Id { get; set; }

    [BsonElement("name")]
    string Name { get; set; }

    [BsonElement("children")]
    Dictionary<string, IChild> StringToChildren { get; set; }
    //above is the issue, changing IChild to Child here and in the
    //concrete works around the issue.
}


public interface IChild
{
    [BsonElement("name")]
    string Name { get; set; }
}

public class Parent : IParent
{
    [BsonId]
    public BsonObjectId Id { get; set; }

    [BsonElement("name")]
    public string Name { get; set; }

    [BsonElement("children")]
    public Dictionary<string, IChild> StringToChildren { get; set; }
}

public class Child: IChild
{
    [BsonElement("name")]
    public string Name { get; set; }
}

{
    "_id" : ObjectId("535a524abfcae300b04cea7b"),
    "name" : "ParentName",
    "children" : {
        "one" : {
            "name" : "bob"
        },
        "two" : {
            "name" : "jane"
        }
    }
}