Writing a play review for children involves seeing broader performances and using simpler vocabulary than writing one for adults --- but don't assume that means it's easier. Children dislike it when adults talk down to them and will not be forgiving if a review states that a sub-par play is good. To write a review, you must pay attention to a variety of aspects of the performance and write in a way that appeals to children without patronising them.
Read a script of the play before seeing the performance, if possible. If you're familiar with the script ahead of time, you are more able to focus on the technical aspects of the performance rather than devoting all of your energy to following the plot.
Keep your playbill from the performance. This helps you remember the names of the actors and characters as well as the titles of any songs.
Take notes on the performances of the actors. Do they underact or chew too much scenery? Do they remember all of their lines? Do characters who are supposed to be funny make you laugh or do their jokes fall flat? Do any actors seem to hog the spotlight?
Note the technical aspects of the performance. Lights, sounds, make-up and costuming should be invisible unless you're watching a fantasy play --- you shouldn't notice that you're watching a play instead of real life. If the play is a fantasy, the costumes and make-up should realistically convey another world and should have a consistent theme (if all the animal costumes are for woodland animals, a lion would be out of place).
Take notes on how this performance compares to your original view of the script. Of course, no director is going to follow exactly what you imagined while reading, but some directors cut characters or scenes, change the setting from one time or place to another or even adjust lines in ways that go against the original message of the play.
Go home and outline your review. Begin with a brief synopsis that describes the premise of the play without spoiling any plot twists. Move on to the acting and technical aspects. Conclude by stating whether you think this play is worth seeing and why. Write this outline as soon after seeing the play as possible to make sure it's fresh in your mind.
Write your rough draft. Use vocabulary based on the age of the children for whom you're writing. If this is a play for third-graders or younger, stay away from words of much more than two syllables and don't focus too much on the technical aspects that went well; very young children don't tend to care about lighting or make-up unless it's obviously bad. If it's a play for fourth-graders or older, use a slightly simplified vocabulary but expound on all of the elements you wrote about.
Revise your rough draft and write a final draft.