Something to keep in mind in Hawaii in general: In the summer, go north. In the winter, go south. This is based on surf - Hanalei waters will likely be rough at that time, although March is late enough that Poipu might not be super calm either. Anyway, go south in March. And keep in mind that there is a completely enclosed beach on the east side which is safe for little ones year-round, with an awesome playground right by it.