In preparing for getting Nathan an omnipod insulin pump, we had to count carbs and calories exteremely close for 6 days to establish a good carb to insulin ratio, I also counted calories figuring I should give it to the CF nutrionist if I was going to have to do such a detailed food log, might as well benefit from both clinics.
Anyways, what I learned is that if you can get 2% chocolate milk, it has 186+ calories to an 8 ounce serving and they won't fight as much, at least Nathan doesn't. Nathan loves his chocolate milk and says it is the same as everyone else drinks and so he often will drink 2 cups instead of just one. It is one of the easier ways to boost calories. I need to check on whole milk with chocolate syrup added because he likes that as well. Anyways, if you use the chocolate milk and just add a 1/4 cup cream and a little more chocolate syrup, they might be more willing to drink it, or drink a little more of the regular stuff. After paying such close attention and learning all that I learned over those 6 days, there is a lot of things that I won't be drinking or eating anymore.
Another idea or suggestion is to get those little ones use to eating a 4th meal before they go to bed. I am not sure how many extra calories a frozen Kids Cuisine Meal has, but Nathan is really liking his Hungry Man Meals before bed, they contain 750 to 1000 calories each. When it was recommended that Nathan eart a 4th meal, I thought are you kidding, no way, but it really works for him to help gain weight, even with his g-tube he needs it to get some gain. TV dinners are the easiest way to get that 4th meal in and they usually love them because they can choose which ones and cook them themselves when they get a little older in the microwave. Creating this habit early on will make it easier when they get older so even if they don't really need those calories yet, it is the habit that you are creating that will pay off eventually.
Just a couple of things that I learned and discovered. Hope it helps. Always looking for realistic, doable suggestions, hope I am offering the same.