First of all, the photo is beautiful. I agree with most of the technical criticisms about exposure, but I really like this photo.
As far as shoots with toddlers go, here's my two cents: I worked for a childrens portrait company for some time (your basic cookie cutter photography... nothing exciting), but one thing that I learned is how to deal with small kids to get the best out of them. A) Whenever possible, use a tripod. It makes it so much easier to get the kids focus when you dont have to worry about constantly moving the camera to frame them. B) To keep them in the frame you already set, it's all about the art of distraction. I've learned throwing stuffed animals in the air and fake sneezing really does the trick, at least for a few seconds at a time, and really, a few seconds is all you need, because C) You have to move fast. You can get their attention, no problem, but you can't alway keep it. Snap about ten times the frames you would if you were shooting adult models.
I hope any of this helps with future toddler sessions!