We played a writing review game that covered the basics they've been practicing in 2nd grade writing. It was super easy to put the Kahoot together, took about a half hour, which is less time than it would have taken to make a paper/pencil review.
Each teacher did ask for the results of the Kahoot when we were done (which Kahoot provides.) Once they get the answers, it's like having a Formative Assessment right in their hot little hands!
Kahoot is SO easy to use for Formative Assessment! Whether you make your own Kahoot or you use one of the thousands and thousands of pre-made Kahoots (as of this moment there are more than 40 million ready-to-use Kahoots!) The best part about using Kahoot for Formative Assessment is the level of engagement soars! The kids are so tuned into the game, and regardless of whether or not they know it's a review quiz or not, they (at least the kids I work with) love playing!
One more tidbit about Kahoot - you can use it as a pre-assessment, too, if you want! Pose the Kahoot in such a way that you know the kids don't know this "stuff" yet because you haven't taught it. You want to see what they know before the unit starts. Then, play the same Kahoot again (ghost mode!) and see how everyone did before the unit and again after the unit!
However you choose to use it, Kahoot is a fantastic (and completely FREE) tool (with upgrades availalbe) that you can use tomorrow. Heck, you could use it today if you're reading this post in the morning!