Decide which child will wear this and then you can fit your large yellow felt circles. I had the bottom of the circle come just to the bottom of my son's knees.
To get the "perfect" circle follow these instructions (you can just do it by hand if you're that good - I'm not). Measure from the shoulders to the knees (my measurement was 36"). Half your measurement (mine is now 18"). Cut a length of string (or thread or yarn) about an inch longer than your measurement (mine string was 19"). Fold your felt in half lengthwise (hamburger style ;-)). Secure on end of your string in the middle (doesn't have to be exact) of your fabric. Secure the other end of your string to a marker. Stretch your marker away from the center of your fabric and draw a circle! (You basically just made a huge compass.)
Cut your circles out.
Now, I like to sew, but you can just as easily use fabric glue. When you join the front and back of your circle, leave a gap at the top big enough for your child's head and a gap at the bottom large enough to not make it difficult for your child to walk. Either sew along the edge or glue it!
For the neck - I made a straight cut so the top of the circle wouldn't bother my son's neck. Also, in the back of the neck I cut a slit about 5" down from the top so his head wouldn't rip the fabric when he put it on.
For the mouth, cut a big black wedge (like a triangle, but the small part of the triangle will be rounded instead of straight). I went from about the middle of the circle to the edge. It is better to cut this too big and trim it later. ;-)
For the eye, cut a small black circle and then cut a small triangle out of it (see the picture).
Secure the eye and mouth by sewing or with fabric glue.