There is something called the Brickmaster Club on Lego.com - apparently if you sign up for it you get a BOGO free cupon for LL. Check the "other theme park" link at the top of the page of the Disboards Disneyland California page, then do a search for Brickmaster.
I don't know all the details, because as Canadians, we weren't eligible, or I would have signed up my
lego fans.
We are taking a side trip to LL during our DL trip, but we're going Greyline (bus tour - don't have to drive), and we've got 11 days total (1 at LL, 7 at parks, one for break, 2 for travel), so we won't be missing out on anything at DL.
I'd definitely add a day - everything I have heard about LL says that your kids will enjoy it (especially your DS) but it's not worth giving up a DL day.