I was hiking with a group from my parenting meetup, and I mentioned one of the snacks I give Violet to help pack on the calories. As a vegan, I do often find myself making a very conscious effort to get lots of healthy fats into my girl. Protein isn't as much a concern of mine because Violet loves beans and hummus and drinks both soymilk and breastmilk regularly, but it is a concern of many parents both veg and omni alike as kids go through picky stages.
I am happy to share some ideas for snacks that Violet likes, but I'd love to hear back from my readers to know what kinds of foods their children avoid. When we went vegan, we had to learn to eat in new and diverse ways. As a result, our diet has so much variety that I don't really notice Violet being picky yet. She could be avoiding certain things, as she throws many meals almost completely on the floor, but since he have few repeats it's not obvious to me. Anyway, my point is that if there are things your child doesn't like, I might have some suggestions of things that have similar nutrients or qualities.
One of Violet's favorite snacks right now is "cookies." Basically it's a raw vegan cookie, the quotes are just because I don't add sugar or bake it. I take quick oats (although I'm sure it would be healthier for me to use rolled oats or other grains- spelt in particular has a nice flavor and is higher in protein), walnuts/almonds/pecans/whatever nuts I have on hand, and dates and blend in the food processor until it's fairly fine and a bit sticky. For extra protein and fat I add flax meal or whole raw sesame seeds (sesame seeds are a really good source of calcium as well). I sometimes add a little cinnamon, cardamom, or other spice to give it some nice flavor. Lately we've been going without spice. Finally, to get a smooth consistency, I blend in some coconut oil. The perportions are different every time and I really just add a little of this and that until it's a gooey consistency. I scoop it out with my little cookie scoop and put it in tupperware in the fridge. These become "cookies" full of protein, vitamins, and healthy fats. It's sweet from the dates and seems like a treat to my little darling. I joke that these are my insurance on those days when it doesn't seem like she's eating anything because I know she's getting a good protein and calorie boost even if she won't eat the other food I try to give her.