When we first moved to the Big Island, we saw this warning on a city sign in a vacant lot. It took us a while to figure out that the pukas were holes in the lava grounds. Sand play and children are a natural. It's easy and fun. But I have had to rescue more than on toddler from a hole that could have easily caved in on the little one.
Sand is heavy.
Keep a close watch on your little ones, even when they're not near the water. Keeping these precautions in mind and invest in a book about Hawaii's beaches and marine life. If you're planning on snorkeling, you can purchase a plastic waterproof guide that will help you to identify the marine life. Hawaii really is a gentle place compared to most. A little knowledge and common sense will ensure you a safe and happy vacation.