Resort recommendation family trip December

I’m looking to spend 5 days in the carribean over my birthday in December. My family is myself, husband, and three kids ages 7-11. Based on my research so far I’ve narrowed the trip to the islands St. Martin, St. Lucia, or Grenada. I’m looking for resort recommendations with the following in mind: – $400 per night or less – on the beach – close to restaurants or restaurants at resort – kids are very picky eaters – smaller, not crowded, would like to spend most of time sightseeing – nice big pool, but don’t need anything like slides or private pools

Open to other islands as well! Thank you.