Protein is a crucial part of a child's diet.  If possible, a toddler should get 16 ounces of protein per day.  While other foods provide good nutrients, it is protein that is the best source for energy.  Toddlers are growing and need every bit of that energy.  There are other foods that will give energy, most commonly sugar, but protein is the best source.  Sugar burns fast, giving a short energy burst, but no positive lasting effects, and nothing to help the body while it is growing.  About 16 ounces of milk and an ounce of meat will often provide the protein needed.  A list of some of the best proteins is provided as follows:

Chicken:  About 9 grams of protein are in an ounce of chicken.
Fish:  About 5-6 grams of protein in an ounce of salmon
White cheeses (cottage cheese):  About 3.2 grams of protein in an ounce of cottage cheese.
Pork Chops:  About 7.5 grams of protein in an ounce of pork chop.
Lean Beef:  About 7.5 grams of protein in one ounce of lean beef.
Tofu:  About 2.3 grams of protein in one ounce of tofu.
Beans:  About 5.2 grams of protein in one ounce of beans.  This may vary with bean types.
Eggs:  One eggs has about 6 grams of protein.
Milk/yogurt:  About 8 grams protein per half cup of milk.
Nuts and seeds:  8 grams of protein per ounce of peanut butter.

This might give you some ideas, especially for those kids that don't like meat.