I agree that Metabolic Drive is a good option, especially since it tastes better than anything else. For a child I guess 1 scoop in some milk, half a banana, ice, and some other stuff would be a good idea.
You could make Metabolic Drive milkshakes by using ice cream. I did this when I had my wisdom teeth out and couldn't eat solid food. Although I lost 10 pounds, I gained it back real fast.
I wouldn't put wheat germ or flaxseed in the shake just for the fact that it might put your kid off the whole shake thing.
Don't worry too much about using a protein shake on your child if you don't see anything wrong with using pediasure or any of those other shakes in a can people give their kids when they aren't eating enough. But if you're against those then think again about using protein powders.