Don't know if this will help, but my daughter was not a twin and was only 20 lbs at 3 yrs old- just a very tiny little sprite. She is still in the 10th percentile but very healthy (She is now 8 yrs old and only about 48 lbs)
It sounds like they eat a very well-rounded diet to me. I would not force feed them. Kids eat when they are hungry and have better instincts about that than adults. Also, remember it is actually better for us to "graze" on small meals than to wait for 3 big ones, so make sure that nutritious things are available and don't stress about it too much. If you're ped is worried, ask why. Are they lower on the charts than they were? My ped says that babies who are high on the charts in the beginning will start to level off to where their adult weight/height will be around 1 yr. So, for example, if you have a very large baby to 2 smaller parents, the baby will start in the 90% but then level off a bit because they may end up being a smaller adult b/c of genetics, BUT... if you have a child who is dropping on the chart TOO much, or who is NOT in line with the parents, then you have a problem.
The questions I would ask are:
#1- do you need a new ped who is more forthcoming with info without you having to beg for it or wonder
#2- have they declined to the point where the weight gain is not enough and something is wrong or ped is worried
#3- what can you do? forcefeeding is not an option, diet sounds good. are you giving enough fats- what does ped recommend? have blood samples been taken- how is iron, etc. to test absorption to rule out things like dairy allergy which can cause lack of weight gain
Go with your instincts above all- if they seem healthy to you, trust that. Remember, to most peds, you are just another number and he doesn't see the whole picture of your children- YOU do.
Hugs,
M.
ETA- even though most peds will disagree, you really don't need dairy in a diet- sometimes it can harm. Most cardiologists recommend NOT eating dairy/drinking cow's milk because it is pretty bad in the long run on your cardio system. Just thought I would throw that radical idea out there.