When talking about this problem with one of my Kindergarten catechists, I decided to make a tool for their classroom to help the little ones learn this important prayer. I once heard a monk say that if you were only going to pray one thing, this would be a good choice because it packs so much in. You've got the Trinity, a connection to our baptism, and a bodily reminder that Christ became one of us and died for our sins on the cross. Seems to me it is worth the extra time to help them get it right.
I started with a cheap mirror- this would be a great project to reuse one you have or pick up one at a thrift shop. I went the easy route and bought a new $5 one. This particular mirror is about 12 x 18 in. You could go bigger, but I wouldn't go much smaller.
I used a white paint marker, and after it dried it seemed pretty permanent and not easy to scratch.
I then thickened up the lines to make it more visible. I added numbers and arrows for non-readers, along with the words of the prayer. When you make this, it is important to have it correct as a reflection, not the actual left to right of the prayer. Make the sign of the cross in the mirror and think about where the 3 and 4 would go.
Next, hang it at a level that the kids can line themselves up with the cross-forehead at the top and shoulders on the horizontal pieces. Then they can look in the mirror, follow the numbers and arrows, and make the Sign of the Cross correctly:
In the name of the Father...
Eventually with practice the motions of this prayer will become ingrained as muscle memory- practicing it the right way will make it stick!
Some other ideas...
-If you have a wall mounted mirror in a bathroom or other place, you could cut out a large cross from the center of a large piece of paper or piece of posterboard. Tape the outside part on the mirror so the kids can still see their reflection in the middle part of the cross. Write numbers and cue words on the paper.
-You also could do something very similar to the mirror above but with tempera paint or window chalk, both of which scrape or wash off, leaving you with a clean mirror once they have mastered the prayer.