When feeding a grain mix, give only a tiny portion at a time, so that the mice have to eat all the whole grains before they get any more. Don’t let them have any more until the grains are eaten, no matter how much they beg – they’ll soon learn to eat properly! If the mix contains pellets, very few mice will eat them – don’t worry about this, as they usually just contain alfalfa for fiber.
Laboratory pellets provide a complete and vitamin-enriched food, but a diet of pellets is very, very boring and since many mice are picky eaters you may have trouble persuading them to eat it.
Personally I would never feed a complete diet like this, no matter how ideal its nutritional content, because mice really enjoy picking around in their food, eating the tastiest parts of a mixture first, and generally being choosy. For a caged animal, an interesting diet adds variety and interest to its life, and it is a shame to deprive a pet of the opportunity to choose elements of its own diet.
If you do use pellets, you can make life more interesting for your mice by supplementing them with small portions of fruit, veg and mealworms.