Well since he is a new cat (new to the family anyway) it may be that he just isn't used to the kids fully yet. It's possible he just had had enough at that particular time and was letting her know it. He does need to learn that his reaction was not ok though. The children need to learn to leave him alone. My in laws have a small dog (yappy little thing that I don't particularly care for) she will tolerate the kids for awhile, then she snaps at them - the kids have learned what her cues are and when she is hitting her annoyance level (if that makes any sense) I don't think it is ok for any animal to lash out at a child, but I wouldn't toss him out just yet, he needs some time to adjust, but I would keep the little ones away for awhile if it were me.