In our hose the hardboiled eggs are the most consumed made ahead protein. You can buy egg-mold for a few bucks, which are little plastic things that press cute pattern on the egg and kids usually love those: bunny, teddy bear, or just heart, stars etc. You cook the eggs at the evening, take off the shell, and put them into the molds in the fridge, and you have cute eggs for breakfast. Somehow if an egg has a bunny face on it , it became yummier
I also like to make colored egg triangles. I mix few raw eggs,a few tbsp thawed frozen spinach in a blender, and fry it in a pan without stirring, so it will be just a plain round green egg "tortilla". Cut into triangles and my son loves it even cold. I may have told him that incredible hulk maintains his green skin color because he eats a lot of those

I make egg fritattas, but my son doesn't like those , but yours might.

I also bake paleo almond lour (mini) muffins. It is not the best food because of the high (natural) sugar content, but it is still waaay batter than any grain baked product, and it is a has the advantage to look/taste like the traditional SAD breakfast. Even my non-paleo husband would it that for breakfast every day if I let him.

Also just various nutbutters with apple slices is a quick filling food. Or "ants on a log " ( celery , nutbutter, and raisins)

What about pancakes made with coconut/almond flour, eggs and almond/coconut milk. Serve with a fruit purée/sauce or bananas/berries. You can premake the pancakes and just warm them through, they can add there own fruit might be messy but fun.
I too really struggle with my children they were both weaned on good healthy fruit and veggies but now just want processed crap. I just made homemade tomato soup and my daughter said it didn't taste like tomatoes it had 8 whole tomatoes and 2 tins of plum tomatoes in it how can it not taste like tomatoes. She means it isn't the same as tinned soup . I despair of them my son eats next to nothing and is so underweight and my daughter only eats processed crap and is piling on the pounds. I will change it though. I try not to stress but it is so hard, I lead by example but it gets me nowhere.

I eat at my office so usually grab something from my fridge as I rush out the door in the mornings - Hard boiled eggs, apples with nut butter, "pancakes" made ahead (I do two eggs to one banana and add cinnamon and/or nut butter) and Larabars are my standbys!