It's never happened to me. You could try trashing the balls, by going to the notebook icon, going to "Present", selecting the item that you want to delete and then when it says "Wrap?" press A until it says "Trash?" and then press B. You could also reset and download. When you press the reset button, press B. "Download" and "Reset" should appear. Choose "Download" to get your Tamagotchi back again.
By the way there are no "unhealthy" and "healthy" foods to make your Tamagotchi's child grow better. What you feed your Tamagotchi doesn't change what it's going to grow into. Some foods, however, raise your Tamagotchi's skill points. For example, feeding your Tamagotchi a Melon raises all of its stats by 1.