What The Robin Said
If your little student never mixes up his 2nd and 4th fingers on at least one hand, you have a very special preschooler. I find that about 100% of my preschoolers have trouble with the 2nd and 4th finger, but most of them eventually get it right. I also get my 2nd and 4th fingers mixed up so I certainly can understand the problems a pre-school child might have. In fact, when I first printed this I noticed that I had reversed those fingers on the little keyboard and I had to do it over again. Because of this reversal problem, I added little blocks to write in finger numbers. If your student is old enough, she can write it in herself. If will also be fine and save some time to discuss it and play various finger games, but you write it in for the student. It depends on the student.
If this is the first time you have seen this kind of music notation, take a look at my Fourth of July blog posting from a few days ago. There I explain how to teach this kind of graphic. One other thing, be sure to encourage students to drop into the keys. They often have trouble doing that on the black keys, but don’t dwell on it.
I have posted several quick worksheets to help students with finger numbers if they continue to have trouble, even after all the activities in their method book. Take a look at Colorful Fingers and Writing Finger Numbers if they need extra help.