Smoothies are a great way to get calories into your child if they are choosing not to eat. My daughter always prefers to drink so if she doesn't eat great during the day I'll make her a smoothie before bed to make sure that she gets all the nutrition that she needs. The great thing about smoothies is that you can put ANYTHING in them.
I like to start with whole fat yogurt and add cream or coconut milk (you could also add oil if you really need fat), nestle nido or formula powder, and any combination of fruits AND vegetables. Yep, that's right... veggies. Remember that your child doesn't know yet what is a "weird" food combination so anything goes. Kale and spinach are really easy to pop into a smoothie and they don't taste very strongly (so you can put them in yours too!), I've also done broccoli and sweet potato... anything that you feel your child is missing.
My basic recipe:
2 oz whole fat yogurt
1 oz cream
2 tbs nido
splash of juice or milk to thin it out
The calorie content will vary based on what fruits you use but on average a 4oz serving has around 250 calories, 18g of fat, 8g of protein. If you use formula instead of Nido it will contain about 180 calories.