WASHINGTON — Grilled chicken replaced the hot dogs. Strawberries instead of cookies at snack time. No more fruit juice — water or low-fat milk only. This is the new menu at a Delaware day care center, part of a fledgling movement to take the fight against obesity to pudgy preschoolers.

Day care is the next frontier: New Harvard research shows few states require that child-care providers take specific nutrition and physical activity steps considered key to keeping the under-5 crowd fit.