Freezing is a go-to technique. Fill muffin cups with yogurt and/or pureed fruit (no sugar added) and freeze overnight. This is a good way to make child-sized portions of meatloaf and quiche, too. As they thaw, these treats will help keep the other stuff in your kid’s lunch box cool at school. Many items — meat, fish, eggs, beans, cheese, milk, rice, pasta, mayo, and butter — need to be chilled.