Here is a great tutorial on closing up those nasty holes that often show up around the base of the thumb when you make a mitten. Please note -- the technique is demonstrated on a fingerless mitten. Those are NOT appropriate for A4A! I'm referring you to this site for the information, but don't leap to conclusions -- A4A wants only mittens that cover the fingers and thumb and are closed at the top.
P.S. I am sorry that I seem to spend so much time saying "no" -- it's just that it's so important to send the kids what they really need. . .