I stress out about my kids getting enough nourishment because it feels like they only want the same stuff all of the time. PB&J is a perfect example. Brant is always asking for it; here is what I did to bump up the nutrition.
Combo 1: Use real peanut butter. We did for a while and then bought a huge container of Skippy at Cosco and and now I have to wean my kids back onto real peanut butter (Krema is my FAV). I do this by mixing together half Skippy, half real PB until in a few days I'm only using the real stuff.
Combo 2: Once on the real PB, I will take a few scoops of PB and put it in a small bowl. I then add a teaspoon of flaxseed oil and mix. I LOVE flaxseed oil. It has no flavor and your body instantly absorbs it. All seeds are awesome for you but your body cannot break it down unless you chew the seeds really well. I have seen whole flax seeds in oatmeal, granola bars, etc. That's great but those seeds will just pass through your body whole if not chewed down properly. Flaxseed oil or flaxseed meal is better because the hard work is done for you, all you have to do is swallow. Same with chia seeds. People are adding these seeds to breads, smoothies, etc. But if you are not grinding them before you add them to your food, your body is not absorbing any of the nutrients. Anyway, I put small amounts of flaxseed oil in their yogurt, smoothies, etc. I started doing this as young as 8mons. A good flaxseed oil will cost you minimum $10 and must be kept cold.
Combo 3: Mix PB and 1 teaspoon of Tahini. Tahini is ground sesame seeds; it's in hummus. Test Tahini first as it, and all nuts really, are a high allergen food. However, they are extremly good for your body - so many nutrients, high in calcium and protein. When I made baby food for my boys, at 8 mons I always added tahini to at least one or two feedings per day. "Seeds are loaded with nutrients. If a seed is placed in the ground, it grows. If a seed is placed in water, it sprouts. Nature must have put a concentrated store of nutrients in the seed, which can grow a new plant with no soil and nothing more than plain water. The seed is life itself." Ruth Yaron Super Baby Food on "nuts, seeds and sprouts".
Combo 4: Mix PB and almond butter or walnut butter, cashew butter, etc. Trader Joe's has reasonable prices on their jars and its worth it if its going to add nutrients to your kids' diets.