Leah, most children ask for a bedtime snack, and this is certainly reasonable. After all, how many of us do this as adults? Children also sleep longer than we do, so they are going more hours without nutrition.
I applaud your desire to make these snacks healthy. Consider puddings, cheese and crackers, pretzels and peanut butter, or anything that has good protein content. We get hungry when our blood sugars (glucose) drop to a certain level. Proteins take longer to digest than fats or carbohydrates and keep the blood glucose level for a longer period of time. Sugary foods or foods with a high fat content, like cookies and candy, will cause glucose levels to rise and fall too rapidly. Whatever they snack on, make sure they brush their teeth well after snacking.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.