I had trouble with supply in the beginning, too, and it totally shocked me because I have tandemed with a toddler and newborn with several of my children. I found that I was not drinking nearly enough water with my twins. I also added a lot of oatmeal to my diet. I would eat two bowls a day until they started gaining well. I've read that it needs to be regular rolled oats, not quick oats or the instant packets. I know your little girl is struggling with latch, but you might want to try feeding them together. If you can get your son latched and the milk flowing, then work on getting daughter latched (extra hands doing this might be necessary). Alternatively, pumping the other side while son is latched can be a great way to produce more for the pump. I have always struggled to pump, but I couldn't believe when I pumped a full 4 oz bottle in minutes when I nursed on the other side.
Sorry, I'm a little all over the board with ideas. You might find you are more successful getting your babies to latch well if you are in the bathtub. Another idea is that they are not too old yet to try self-attachment. Video: http://breastcrawl.org/video.shtml fast forward to 2 min mark, and note that this might take 30-60 min to accomplish this. I noticed a marked improvement in latching after doing this with one of my twins. The other didn't succeed, but after noting that time stated in the video, I'm realizing I didn't wait nearly long enough! There is a lot of information at that website, too.
Also, do you have a good twin nursing pillow? I found mine invaluable in the first weeks. I had a few other thoughts, but now they are gone. I'll post back if I remember them. Darn twin-fatigue! LOL