public interface IEntity
{
    string Id { get; set; }
}

public class Parent : IEntity
{
    [BsonId]
    public string Id { get; set; }

    [BsonRepresentation(BsonType.ObjectId)]
    public string ChildId { get; set; }
}

public class Child : IEntity
{
    [BsonId]
    public string Id { get; set; }
}

var children = GetChildrenQueryable<Child>(db, "Child");
children.Count();

private IMongoQueryable<TChild> GetChildrenQueryable<TChild>(IMongoDatabase db, string childCollectionName) where TChild : IEntity
{
    var parentCollection = db.GetCollection<Parent>("Parent");
    var childCollection = db.GetCollection<TChild>(childCollectionName).AsQueryable();

    return parentCollection
        .AsQueryable()
        .Join(
            childCollection,
            p => p.ChildId,
            c => c.Id,
            (_, c) => c);
}