Hey there Harold,
Welcome to planDisney and thank you so much for your question! What a fun tradition you've started with your niece - the Mickey Mouse Celebration Cakes are a really fun (and delicious) way to celebrate a birthday.
According to the
Cakes at Disneyland Resort page, you have the option of pre-ordering a celebration cake for delivery to select table-service restaurants at Disneyland Park, Disney California Adventure Park or a Disneyland Resort Hotel. Currently, the participating restaurants are
Blue Bayou Restaurant,
Café Orléans,
Lamplight Lounge,
River Belle Terrace, and
Wine Country Trattoria. As of the time that I'm answering your question, this would be the way you'd need to order your cake.
Additionally, since there are limited quantities, I would highly recommend adding the cake to your dining reservation at the time you book it!
Beyond your celebration cake, make sure to also pick up a
complimentary birthday pin for your niece! She will get well wishes from Cast Members and Guests alike throughout your visit. You can pick these up from most check-out counters at merchandise locations around both parks. Also, don't forget a balloon! Everyone deserves a balloon on their birthday.
Thanks again for your question, Harold. I hope you have a wonderful visit to the Disneyland Resort!
Have a Magical Day,
Greg