I'll give you the physician perspective, but also from the perspective of a mom of 2 and aunt to a tiny girl who also failed to gain weight.
1. If your child has dropped 2 % curves on the growth chart, she needs lab testing. After all, if there is a treatable problem, wouldn't you want to know now? Labs include kidney function, blood count, urine testing, thyroid, CF sweat test, and any others needed. I have found kids with treatable problems who then thrived.
2. Nutrition: See a dietician for recommendations.
For all kids: Toddlers need 3 meals and 2 snacks daily. Grazing is not recommended because it sends the body the wrong signals and can cause long term food related and weight issues. Calories per day should be around ###-###-#### or more, but only a dietician can calculate specifically. Milk should be whole milk until at least age 2, I recommend age 3, and 16-20 ounces per day. At this point, breast feeding may be a supplement, but you may want to offer whole milk at meals. Other liquids mostly water, 2-4 ounces of juice at most (empty calories). Concentrate on healthy foods. Offer a variety at each meal and snack, some protein source, and at least 1-2 foods you know your child will eat. Do not fret if she does not finish everything on the plate or even skips an entire meal or eats the same thing every day, seeming to survive on yogurt and fruit. All toddlers do this. A portion is 1 tablespoon per year of age. You decide what and when the child eats, he decides how much and can get more if wanted. Use dips, cut food in sticks to pick up (even sandwiches, french toast, etc) and make it fun, and add nutritious calories, no junk food.
3. I do not recommend pediasure. You can make your own smoothie with much better ingredients and no added sugar - whole milk yogurt, fruit, soy milk, peanut butter or almond butter, etc. Remember this is a supplement or snack, not a meal substitute. The more sweet stuff you give kids, the less healthy stuff they will eat.
4. Toddlers don't really need restrictions on fats, but the fat calories should be healthy, naturally occuring like in nuts, avocados, olive oil, flax, whole dairy products. Even a child with poor weight gain does not need empty calories or junk food.
5. Remember, most kids who don't gain weight as the growth chart predicts do fine and have no health problems. My niece is now 7yo and thriving, but still small and thin - it's just her build.
Good luck and really consider a pediatric dietician asap!