the turnstiles open at 9 and the Imagination Zone opens at 9:30 and the rest of the park opens at 10. From 9-10 hotel guests are allowed in the park and can use select rides scattered around the park.
They don't do a bag check at LEGOLAND so going in can be fairly fast. I'm guessing at 9 they let you into the area called "The Beginning" where there are stores, guest services and a place that sells food.
I'd say get there by 9 to go in first and have a game plan on what you want to ride. Hotel Guests will likely already be in line for some of the more popular rides.