Ok so my daughter was at one point down in the 4th percentile when she was 2 months and we struggled to get her weight up until she started solids. I studied nutrition and have done tons of research on baby nutrition, so I did my best to incorporate that into her food as she grew, and at her 12 month? 74th percentile for weight! So here's what I do: Breakfast - scrambled egg cooked in 1 tbsp avocado or coconut oil (mix in some chopped veggies and cheese if baby likes!) Snack - Greek yogurt with chopped fruit Lunch - diced dark meat chicken (most nutrients) in creamy mushroom sauce (just Sautee a little butter and cream together with mushrooms, pinch of flour to thicken, it doesn't have to be fancier than that) Snack - banana drizzled with melted peanut butter and a small amount of honey, or avocado with some chopped tomato and sprinkle of cheese Dinner - lentil pasta with red sauce and parm and salmon cooked in avocado/coconut oil Late Snack - 1-2 oz cheddar cheese She also get whole milk 2x a day
So the keys are using healthy oils to cook with, adding things like cheeses and nut butters (watch for added sugars tho), not shying away from meats and healthy carbs/whole grains. I like sneaking veggies in where I can cuz she needs those of course, but like, if she's gonna have broccoli I'm gonna put some butter and cheese on it. If she's gonna have fruit I'm gonna put some yogurt or nut butters on it. If I have to cook something for her I'm gonna use more oil or butter than I would for myself. If she wants a snack idc how recently she's eaten, she's getting a snack lol
And of course the menu I gave you is just an example, we mix up foods constantly and she is so not a picky eater which makes things really easy! I also tend to make her food in 3 portions so I can have leftovers for the next day or two and only really cook one meal a day for her that way!
High calorie ideas: cream cheese toast, avocado toast, peanut butter toast and PB&J toast. Whole fat Greek yogurt mixed with peanut butter and berries, sour cream and mushroom pasta (stroganoff-ish), tomato mayo sandwiches, cheese quesadillas, scrambled eggs with cheese and veggies, grilled cheese with sliced tomato inside… You can always add avocado, whole fat yogurt, nut butter, hemp seeds, olive oil, heavy cream and/or chia seeds to meals to boost calories. You could also do a smoothie for snack with lots of those above ingredients plus fruit for a calorie boost. You could also look into the toddler nutrition drinks.