Set a good example. Your kids need to respect their mother and you need to both set the example and have the expectation that this is so.
Presenting a united front with your kids' mom is very important and one of the best ways to do this is to ensure a respectful attitude is always expected toward her. Stick up for her, she’s your bride. You want the same for your kids later in life too. Show them what it means to be loyal to your spouse, so that they will pass on the favor later.
Might I add that this is true for you if your kids mom doesn't live with you? Far better to pass on that attitude than a bitter sarcastic one.