M.
My daughter is 4y old now but has had a hard time gaining weight. She was a preemie and was weighed frequently once we got home. At 2y she still weighed about 21-22lbs. At 3, she was only at 25lbs. Our pediatrician was not too concerned because she was gaining on her growth curve. She did say that yes weight gain does slow down at this point and calmed my fears by saying that someone has to be in the 3rd percentile or it wouldn't be there. I felt a bit better. She also recommended 1-2 pediasures/day if she wasn't eating well. She said don't let it take the place of a meal. Pediasure is 30 calories and ounce and whole milk is about 20 cal/oz and can be very filling. What I would do, is watch what she ate during the day and if it wasn't much, I would give her a pediasure after dinner. She wasa weird kid and actually liked it. We served them very cold, right out of the fridge.
Personally, I wouldn't start adding in a bunch of sugary stuff--they may add on some weight, but it will also start some bad eating habits, that will be hard to break later. I would try switching to the whole wheat, whole milk, and using butter (this is stuff I heard from a feeding specialist before).
I know it's hard not to worry, but look at her development as a whole--is she meeting her milestones? Is she active and otherwise doing well? If so, she may just have a really good metabolism. My daughter finally put on some weight from age 3 to 4--about 5 lbs. I think she hit the 10th percentile for her age--a first for us. Also ask the pediatrician for a referral to a dietician or a nutrionist for other healthy ideas.
Hope this helps.
M.