Hi N.. I am a Mom to two boys - ages 5 (will be 6 in Jan) and 3 (will be 4 in May). My boys are at the 50% in height, but weight is 10%. I have worried about this for 2 years, and last week met with my peditrician about my concerns. Of course, neither are big eaters, are very picky about what they like, and refuse most healthy food and snacks. As infants, they ate almost anything in the jars, but as soon as they could chew and "decide" what to eat, their choices became minimal. My 3 year old still wears 2T pants, and my 5 year old wears 4 slim pants (and they barely fit but he needs the length).
I asked our doctor about supplements, calorie drinks, super vitamins, etc. In a nutshell, she said to do nothing like that. She said that unless they were actually choosing to starve themselves (they're not), kids won't let themselves starve, and that as Moms, we need to look at their intake on a weekly basis vs a daily basis. If they get 3-4 servings of fruit in a week, that needs to be our measure vs checking the food groups off daily. She also said to be sly....like make them pumpkin muffins from breakfast b/c there's real pumpkin in the mix. Or fruit smoothies and call them milkshakes. Or oatmeal chocolate chip cookies, b/c at least they're getting the oatmeal.
Other tips she gave was to serve food in 3 ways at a meal: one thing you know they'll eat, one thing they'll likely try and eat, and one thing you want them to try. We've done that for 2 weeks now and we're making progress.
And finally, not to fill up on Vitamin D milk (which they were). So we limit milk to 1 cup of 2% milk for breakfast and another cup before bed with a snack. That way, they'll desire food, and not fill their stomachs with milk calories.
If you're regularly going to your doctor and talking about your concerns, they should be able to help you and also recognize anything abnormal. But I suspect if you talk to most Moms and doctors they'll tell you kids are all individuals and that their age doesn't dictate their clothes size.
Good luck to you!
A little about me:
Recently, I became a stay-at-home Mom of a kindergartener and a 3 year old.