For the Captain America shirt I did reverse applique layer by layer starting with the star first then working my way outwards. It was pretty easy and I loved the look of it so much that I made one for my little one as well.
I didn't know how to do an Iron Man shirt at first but suddenly the idea just came to me. I sewed a circle of plastic to the inside of the front of a red shirt. I then cut away the excess of the shirt (reverse applique basically). Then behind the plastic I sewed really thick white interface material but left an open at the top so that glow sticks can be inserted in. Basically it's a pocket with a see throw front. And voila, he is Iron Man
For the Thor shirt I used the good old freezer paper and paint technique.
For the Thor font you can download it HERE
Below is the image I used for a pattern. I wish I can remember where I found it, but I can't seem to find it again.
These boys are definitely super heroes in the making