A Chipmunks diet is fairly simple, many pet shops do commercial mixes for them such as Supa Chipmunk muesli with particular favourites being items like hemp, millet and monkey nuts. Diet for a healthy chipmunk should also include fresh vegetables, fruit, flowers and insects such as crickets, locusts and mealworms.
They must have constant access to fresh water. More than one source is best to avoid arguements and if using bottles, in case of one being chewed through. They also will hoard food so you will need to check nestboxes and remove uneaten fresh food to prevent spoilage.