Nutrition Facts

One of the hardest parts, for me anyway, about trying to fatten up my child is also making sure she eats a balanced diet. I know she needs fat and calories, but she also needs fruits and veggies which are lacking in both the fat and calorie department. If your child is still on formula or breast milk you don't need to worry about the variety of food that they eat because both are complete nutrition and any solid food that they are getting is a bonus to their diet. However if you've made the switch to whole milk it's important that they get their vitamins from their food.
Serving sizes for toddlers are much smaller than those we eat as an adult, though the food pyramid is basically the same. For toddlers 1 serving is 1 tablespoon (1/2 ounce) per year of age, and on a daily basis they should be consuming

• Fruits and Vegetables: 4-5 servings
• Protein: 2 servings
• Dairy: 16-24oz
• Grains: 4 servings

As important as it is to offer a varied diet, it's also very hard to get it all in when your child doesn't eat much. Don't stress about what your child is eating each day, instead try to concentrate on what they are eating over a week. Offer a different fruit and veggie with each meal and try to vary the colors of food that you offer.
A couple other important meal tips... don't let your child snack all day if they are trying to gain weight. It's important that children learn hunger cues and spreading their meals out helps them eat more at each meal. Offer 3 full meals and 2 snacks during the day, serving milk with meals and snacks. Try not to offer juice or water as you want everything that goes into their stomach to have calories but also don't let them drink a ton of milk. If your child is filling up on liquids they aren't going to want to eat and food has more nutritional value than milk (the same does not go for formula/breast milk). Offer all meals at a table, it's an important that your child get used to meal times and the more often you can eat as a family the better! And don't make meal time a battle. I know it's hard to want to force them to eat, but you don't want them to have negative associations with food. Respect their cues, if they are done then let them be done.
One last thing. I've seen it mentioned many times that a 1 year old should be consuming about 1000 calories a day. PLEASE keep in mind that is for your average toddler and not really for kids who are underweight. A good guideline is to shoot for 50 calories per pound, per day.