I'm not a nutritionist but I was a nurse, and I have a strong personal interest in nutrition. Things my two kids enjoy (inconsistently, LOL):
-Smoothies, with Greek yogurt, nut butter, or hemp seeds for protein. I don't do protein powder with them, and would advise against it unless the kid is vegan or vegetarian.
-Greek yogurt with fruit and granola
-Oatmeal
-Turkey or chicken sandwiches
-Pasta with meat sauce (I try to use whole wheat pasta when possible)
-Lentil dishes
-Beans! You can do a TON with chickpeas and black beans
-Edamame
-Energy bites/balls made with nut butter
-I found some almond based pudding pouches at Sam's Club recently that they will grab on the way to practices, decent amount of protein and shelf stable
-Eggs (one kid hates them, though - my gymnast)
-Vegan baked goods - I'm the vegan in the house but many vegan recipes for muffins and pancakes and such can have good amounts of protein, depending on ingredients used
Nuts and seeds (especially hemp seeds) are also something I sprinkle on certain foods for an extra boost of protein. Again, something from plant based eating that I apply to their diet.