In the first machine applique class I attended, we were told never to use 'monofilament' or polyester invisible thread on kids quilts especially if the quilt is for a little boy. Sometimes through wear the thread becomes loose and can become wrapped around fingers and toes and "other " parts. We may not always notice loose threads because they are invisible, as they were meant to be. This is not meant to be a joke or to scare anyone but the consequences of monofilament wrapped around little toes could be disastrous.
I have made several kids quilts and have had good luck matching the top thread to the applique piece. Hope this helps.

I had started this thread here on quilting board site

http://www.quiltingboard.com/t-35487-1.htm