I've used this a few times, and while it is good for alerting you of passive voice as well as a few other things, as far as these kinds of apps go I prefer Grammarly, and would suggest it to others as my top pick for grammatical assistance.
This is part of the free editing set from First Grade Wow. For kindergarten, I use it to show what they can do (smiley face in the check box) and what their newest personal challenge is (I highlight the check box). They keep it in their writing folders. It helps differentiate learning and gives each student focus. There are many other useful pages in the packet, too. All free!