One interaction at a time. Just make the next interaction a relationship-building one. Don't worry about the one AFTER that, until IT becomes "the next one."
Helping a child learn to hold a kitten or a puppy isn't always easy, but modeling how to do it gently and softly helps. Parents can remember those factors when touching babies and children, too. Is he comfortable? Is he safe?
Someone who can gently handle a puppy, and a baby, might remember those things when dealing with another adult.
Life *is* frustrating. Being mindful won't prevent kids from getting frustrated but it will be a huge step in the right direction. Seeing the world from kids' point of view will help you understand why they are reacting to the world as they are. Treat your kids as though they're doing the best they can with the knowledge and skills and understanding of the world they have. And often when they're at their worst, what works best is a hug.