Selecting an embedded object using LINQ (from MongoDB)

I feel like this must be simple to do but can't get it done - any help would be greatly appreciated. I am relatively new to LINQ and MongoDB.

I have the following two entities:

class ParentObject
{
    GUID ParentId {get;set;}
    IList<ChildObject> ChildObjects {get;set;}
    ...other properties

    ParentObject()
    {
        childObjects = new List<ChildObject>();
    }
}

class ChildObject
{
    GUID ChildId {get;set;}
    ...other properties
}

That I'm persisting to a MongoDB instance (as one document, with the subdocuments embedded). What I need to do is retrieve only one of the ChildObject sub-documents stored within the ParentObject document using the ParentId and ChildId. I have the following method/method call that almost does what I want but I'm only able to return an IList or one specific ChildObject based on the index (o, 1, etc):

// I know this won't work    

var parentId = (Some GUID);
var childId = (Some GUID);

var result = 
    SingleWithSelect<ParentObject, ChildObject>
    (
        x => x.Id == parentId && x.ChildObjects.ChildId == ChildId, 
        y => y.ChildObject  
    );


public TResult SingleWithSelect<T, TResult>(
    System.Linq.Expressions.Expression<Func<T, bool>> whereExpression,
    System.Linq.Expressions.Expression<Func<T, TResult>> selectExpression)
    where T : class, new()
        {
            TResult retval = default(TResult);
            using (var db = Mongo.Create(ConnectionString()))
            {
                retval = db.GetCollection<T>().AsQueryable()
                            .Where(whereExpression)
                            .Select(selectExpression)
                            .SingleOrDefault();
            }
            return retval;

        }

Many thanks in advance for any help/pointers.

-Mike