My family and I stayed at the Navy Lodge North Island (NASNI) on Coronado Island near San Diego, CA over the Christmas season 2017. I highly recommend it for a stateside sandy beach getaway in December. Don't expect Hawaii 90* weather days, but we still enjoyed a couple nice walks on the beach and sunny days out on the town.
*Travel Tip - The San Diego Zoo is free entry with an Active Duty ID. You don't even need to go stand in the ticket window line, just go straight to the entrance turnstiles and show your ID.
My father (Army) booked the location about 10 months ahead in person while on exercise there. He wanted us to all stay together and so booked a beach cottage for $140/night.
From their site "Breakers Beach is the proud home of twenty amazing new beach cottages. Relax in a casually elegant two-bedroom cottage that includes a fully-equipped kitchen, flat-screen cable TVs with a Blu-ray/DVD player, WiFi Internet access, linens, a private bath, patio and BBQ. These single-story units are right on the beach and have stunning views of the Pacific Ocean."
It seems as though the Navy is trying to keep these cottages a hidden secret since it's really difficult to find any good information on them or how to book (The online search options only showed me rooms at the main hotel). It doesn't seem like you can book them online so I recommend calling the standard navy reservation line (1-877-628-9233) and specifically ask for "The Navy Lodge Beach Cottages on North Island".
Side note - I just did a quick search of the standard hotel which you can find a 2 queen bed room for $104/night and a family suite for $136/night. (Note the famous Hotel Coronado is only a 15 minute drive away and family cottages start around $350/night)