Look! The children _______ (play) in the snow.

The sentence requires filling in the blank with the correct form of the verb 'play' in either the Present Simple or Present Continuous tense. Since the sentence starts with 'Look!', it indicates that the action is happening right now. Therefore, the Present Continuous tense should be used. The Present Continuous tense for 'play' with the subject 'The children' is 'are playing'. Thus, the completed sentence is: Look! The children are playing in the snow.