OK this is the least original tip in the world—but it is also the best. The surefire answer to many issues with kids under 9 directly ties to food (it may be for those over 9, too—but I only cover up to 9 so far). We learned to always start with food as the solution. If you let too big a gap occur between a meal and a snack or the next meal, you are done and likely facing a catastrophic meltdown that could test your sanity. The solution—just keep pumping healthy or at least reasonably healthy food into your kids and your life will be bliss—till they become teenagers or so I am told.
