It's time to take exercise with the children.
It's time to do sth是固定结构
exercise在解释为锻炼、运动时是不可数名词,一般写作take exercise当它解释为练习、习题时是可数名词,如做一些练习题就可写作：do some exercises还有做眼保健操时也一定要写作：do eye exercises
在这里,根据语境,应该是说现在到时间和孩子们做运动了.