It's funny you say you were aiming for 22 pounds at 2, so were we! We didn't make it either
Connor was barely over 20 pounds at his second birthday, he had been 21, but then got rota virus and lost a bunch of weight, and it took him months to gain it back.
Periactin was brought up for him too, but his intake is not the issue (he does eat!). AND, his syndrome is known to be associated with slow gastric emptying, constipation, reflux, etc and all of those are exacerbated by cramming larger quantities of food in the kiddos. It used to be pretty standard course to give all kids with his syndrome g-tubes and try to get them to grow faster that way, but the medical community is finally catching on that it's leading to a cascade of problems with the slow emptying, constipation, reflux, leading to nissens, enemas, etc etc etc.
In Connor's case, once I finally got ahold of the growth charts for his syndrome, and when we took into account his hypotonia, his weight really isn't that bad. Yes, he's small, but his height/weight ratio isn't "that" far off really, and like I said considering his low muscle mass it makes even more sense.
Interestingly...when I was talking to the Dev Ped about this (it was GI that wanted him to try Periactin) the Dev Ped kind of rolled her eyes and said "GI wants everyone to fit in a box, and even wants my spastic CP kids, who will never be on a growth chart, to gain weight and it just isn't reasonable."
So we didn't do it. Obviously difference circumstances could mean that it's worth trying, but when taking the big picture into account for Connor, Periactin wasn't the answer. Have you plotted your daughter on the CP growth charts? Our Dev Ped said something about those charts, so they must exist. Maybe 20 pounds is reasonable.