My dd is 12 years old and has always weighed less than average. If you look at the growth chart over time you would see that she gained weight and height at a normal steady rate along the same line just at the lowest percentile for weight. There is absolutely nothing wrong with her. She is just smaller than the average.
I've never fed her a bunch of unhealthy fats or sweets to try to get her to become average. I think that is poor advice and you are right to ignore it.
We do not give dd low fat or fat free versions of products. She gets regular milk and cheeses.
I pretty much let her eat when she is hungry and offer a variety of foods.
She likes fruit, cheese, crackers, granola bars, yogurt, chicken or turkey, eggs, some beans, peanut butter on crackers, pbj, bean and cheese burritos, cereal, mac and cheese, fish, oatmeal, meatballs, hamburgers, cottage cheese, pizza, quesadillas, Carnation Instant Breakfast, fruit and yogurt smoothies, mashed potatoes, ham, pork chops, eggrolls, gyros and pasta. She prefers egg whites only. She doesn't eat every kind of meat in every form.
If your your children will only eat small amounts maybe they would do better with several snack sized meals. When my dd was younger it seemed to help her feel less overwhelmed to see less food at once. I would always give her more if she was still hungry.
We have a cookbook called Good Food for Kids by Dr. Penny Stanway and dd enjoyed many recipes from that book. It has information on meal planning and what portion sizes should be for different ages as well which might be helpful. www.cookbookvillage.com/products/good-food