Hi, Theresa!
Welcome to the Disney Parks Moms Panel, and thank you for your question. Yes, you can purchase theme park tickets at the front desk of Disney’s Grand Californian Hotel & Spa upon check-in. Tickets are also available at the Disneyland Resort Main Entrance Ticket Booths just outside the parks.
As you mentioned, Guests of the Disneyland Resort Hotels can take advantage of Extra Magic Hour early theme park entry. The current Extra Magic Hour schedule allows registered Guests early access to Disneyland Park on Tuesdays, Thursdays and Saturdays and Disney California Adventure Park on Mondays, Wednesdays, Fridays and Sundays. If you arrive on your check-in day early enough to enjoy an Extra Magic Hour, you are welcome to do so. Your room may not be ready at the time you check in, but you will be given a room key that you can present to enter either Disneyland Park or Disney California Adventure Park before it is open to the general public. You are able to enter one of the parks early on the day you check out, as well.
I hope this helps, Theresa, and that you have a perfectly pixie-dusted visit to The Happiest Place on Earth this spring! Please write back with more questions as your planning continues.
Wishing you magic,
Courtney