As a parent, it's impossible to keep your cool 24/7. There's only so much whining and tantrums a person can take before she snaps—even Supermom loses her temper from time to time!

When your kid's very young, your words aren't as important as your actions—but as your child ages, what you say has a huge effect on his self-esteem. Words can't be deleted like a bad comment on Facebook, so it pays to speak carefully. Child experts reveal what not to say to your kids, even when things get heated.

"I hope you don't end up like..." Focusing on what you don't want your child to become can be as detrimental as insulting him. "We get whatever we focus on, and whatever we focus on expands," says clinical psychologist Nancy Irwin. "If you suggest to kids that they are shy, fat, slow, just like their father ... it will become a self-fulfilling prophecy."

  • Instead: Focus on positive traits that you'd like to pass on to your kid: creativity, sense of humor, smarts, curiosity. Work on improvement instead of setting him up for failure by exploiting his weaknesses.

"You can't..." While you might preach realism as a parent, telling your little one that he simply can't do something that he's attempting is crippling. "Children revere what their parents think of them," says Tammy Gold, a parenting coach. "A simple insult could crush the self-esteem of a child."

  • Instead: Reserve restrictions to when your kid's safety or behavior is at risk. Otherwise, it's important to nurture exploration as part of a healthy childhood.

"Never." Family psychologist Edie Raether takes a hard line against negative parenting. "Never say never! That is the most important rule, as we all need to dwell in possibilities."

  • Instead: Give your child options. For instance, if he wants to take up BMX racing and you think it's too dangerous, what are some other sports or activities he could try instead?

"I'm gonna kill you!" OK, we've all had that moment where terrible words have slipped out. But using violent threats—kidding or otherwise—is never okay, according to school social worker Devra Gordon Renner.

  • Instead: Pay attention to how what you say is perceived by your child. You've had years to understand sarcasm and voice tone. Your little one? Not so much. Swap violent threats for real, healthy discipline. A time-out would be infinitely more appropriate.

"You are the reason why..." Unless you're going to finish that sentence with a positive statement, don't start it at all. Don't play the blame game. "These types of comments destroy a child's sense of unconditional love, place unfair and inappropriate blame on the child for adult issues, create insecurity and destroy self-confidence," warns child psychiatrist Tia Horner.

  • Instead: Take responsibility for your own adult problems. Your child is not to blame for an empty bank account or a bad divorce. Make sure you build him up by cherishing him, not making him feel guilty.

"Stop acting like a baby." When your 5-year-old goes into tantrum-mode at the grocery store, it's tempting to throw his yearning to be a "big boy" in his face in an effort to make him stop. Ridiculing or shaming him into better behavior isn't okay.

  • Instead: School psychologist Tina Feigal has a better idea. "If the child is regressing, look for a stressor that may be the cause and try to alleviate it. Don't blame the child, as he or she is not doing this on purpose."

"I wish you were more like your brother." "The comment I have heard most frequently that seems to haunt adults from childhood is a reference to not being wanted," reveals Julie Gurner, a clinical psychologist who specializes in adult psychopathology.

  • Instead: Make sure your child knows that he's loved as an individual. Even if he's acting out or misbehaving, treat the issues at hand; don't show favoritism by wishing away the qualities that make him unique.

"I'm disappointed in you." Hearing these words can be absolutely devastating to the fragile confidence of a child. Children want nothing more than the approval of their parents or the acknowledgment of a job well done.

  • Instead: Clinical psychologist Jennifer Powell-Lunder suggests another method for expressing disappointment: "Instead try: 'I am disappointed with the choice you made' or, 'You usually make better choices; what do you think happened here?'" It shows that while you're disappointed in poor behavior, you still love your child unconditionally.

"You're a bad boy!" When your little one acts out, this label can tumble out of your mouth without you even realizing it. "Their behavior might be bad, or the choice they made was bad, but your child is not bad. You need to understand the difference, and so does your child," warns marriage and family therapist Lori Freson. "I can guarantee you is that if your child believes he is actually bad, his behavior will be even worse."

  • Instead: Treat the behavior, not your child on the whole. If he has a tantrum at the library, tell him that being noisy in the library isn't a good choice, not that he's inherently bad for making a fuss.

Look, there's no perfect parent in the entire world who doesn't lose her cool every now and again. And while it's totally fine to feel annoyed, angry, tired, disappointed or upset with your child, the important thing is that you know what not to say to your kids. Of course, even if you do make a mistake, an apology and a hug go a long way in making sure your child feels loved and supported.