All toddlers, not just the ones who are lower on the growth curve, need fat in their diet to help them grow. In fact, fats should make up half of a baby and toddler's daily calories until the age of two for proper brain and body development, so it's important to include healthier fats at most meals and snacks. Fats help our bodies absorb crucial nutrients like Vitamins A, D, E, and K, and are key in building our central nervous systems. But like all things with feeding toddlers, this can get tricky if (ahem...when) your little one has strong opinions about what they eat! And, of course, it's also too easy for us parents to obsess about whether our kids are getting the nutrients they need.
(Quick side note: It seems common for pediatricians to tell parents that they need to "fatten up" their toddler, so if that happens the first thing I'd check is whether your child is on their own growth curve. If your little one has always been in the 5% percentile for weight and haven't suddenly dropped off their curve, they might be growing exactly how they are meant to. And if they eat and drink well, have lots of energy, and your gut tells you that they're genetics are at play with their body type—maybe they look exactly like you did as a child—then I personally wouldn't go too far off the ledge with adding extra fats into all of their food. Talk to your doctor about your specific concerns if this is a situation you find yourself in. And remember that the growth curve goes from 0-100, and kids that fall anywhere on that curve can be healthy!)
The key with fats and kids (and all of us really) is to focus on the healthier ones—which generally speaking, are the ones found naturally in foods like nuts, seeds, plants, fish, and grass-fed meats—and less on ones that show up in packaged foods. This follows my general approach to feeding: Surround your family with more fresh, whole foods, and less packaged snacky ones when possible, but don't make yourself crazy doing it.
You can study up on all of the different kinds of fats and know the ins and outs of which ones are better for you and your kids if you want to, but I find that that approach can be confusing for many people. Instead, I like to focus on actual foods, rather than tunnel-visioning on macronutrients, since it's much easier to wrap my head around what to buy at the store. I don't have to remember the difference between omega 3 and omega 6 fatty acids or what ALA means. Instead, these are the types of fat-containing foods that I like to feed my whole family.
Beef, grass-fed if possible
Cheese (shredded, sliced, cheese sticks)
Chicken, dark meat
Coconut, unsweetened shredded
Cottage cheese, 4% fat
Sunflower seed butter
Yogurt, full-fat plain
Wild salmon, fresh, frozen or canned
Besides the dairy, meat, and fish, I think of everything on this list as a garnish and add it in smaller amounts to the foods that we eat. This adds a range of nutrients, including fat, and helps to make the meals that we eat more filling and satisfying. (And, yes, delicious.) Of course, you don't need to stock all of these foods in your house at once and there will be days when your toddler only wants the peanut butter or cheese or yogurt and that too is okay. Try to keep the bigger picture in mind and think of their intake over the course of a week, rather than bite-by-bite. Buy different foods at the store from week to week, like chia seeds one week and sunflower seeds the next, to easily add in some variation.
I add a small amount (usually 1/2-1 teaspoon) chia, flax or hemp seeds to smoothies, overnight oats, and oatmeal. We have peanut or sunflower seed butter sauces over noodles, rice, or veggies. I regularly make Thai curry with coconut milk, or I sometimes also use coconut milk as a base in soups. We spread nut and seed butter over toast and pancakes. I roast veggies in olive oil, saute in olive oil or butter, and buy whole milk and dairy products whenever possible. And my girls love freezer pops made with a blend of equal parts plain kefir and mango chunks.
We also eat grass-fed beef when we can get it (and when I can find it at an affordable price) since it tends to have a better ratio of healthier fats. And I try to make fish a few times a month, though that tends to be a challenge where we live since there aren't always great options. (Salmon cakes continue to be a go-to meal for us though.)
This of course is not a completely comprehensive list, but hopefully it gives you a place to start if you've been concerned about this topic—and if your toddler has been refusing some of the healthier fats you've been offering him.
P.S. Schools and many conventional pediatricians still recommend switching to low-fat or fat-free milk after a child turns two, but you may not want to do that. A 2013 study found that kids who drink low-fat milk tend to be heavier. And a 2016 study found that kids who drank milk with higher fat content had higher levels of Vitamin D and lower BMIs. Just something to keep in mind!
This is not meant to be a substitute for medical advice. Always contact your pediatrician as needed.