Establishing independence from our parents is a life-long effort for most of us, but it can be done, often with a minimum of fireworks or tears. Try following this 4-step process.
1. Say what you observe happening, in clear, non-judgemental language. "Mom, I heard you tell Suzi to stop (some behavior) four times in the past ten minutes." Or, "I noticed you took Suzi's toy away from her." Or, "You gave the girls a treat I wish they didn't have." Or, "You have given Suzi instructions that directly contradict my wishes."
2. Make a stab at what her motives might be. Use the most compassionate language you can. "I wonder if you worry that she will make a poor impression on people." Or, "It seems that she is really bothering you." Or, "It looks like you're concerned about her developing a bad habit." Or, "You really want Suzi to love you." Or, "It appears that you worry about my parenting skills."
3. Tell clearly how you feel about it (not "you MAKE me feel negative," but rather, "when you do this, I FEEL negative." A fine distinction, but empowering). "I worry that my daughter will be confused about who's in charge here." Or, "I feel angry / hurt / upset when you take issue with my parenting indirectly." Or, "I'm anxious about both being a respectful daughter to you and a responsible parent to my children when this conflict arises." Or even, "When you do that, I sometimes wish you weren't in my home." Or, "I'm sad that my daughter seems anxious to be left alone with you."
4. Tell what needs you have that are being violated or ignored. "I have looked forward to raising children according to my own wisdom, and I need your cooperation / support." Or, "These are my children, and I need you to respect my choices / style / parenting." Or, "I need you to consider my feelings / needs / wishes here." Or, "I want to relax and enjoy having you here."
5. Make a clear request that is within your mom's ability to satisfy. "So, I want you to pay attention to my rules when you are interacting with my children." Or, "Please check with me first to be sure that doesn't mess up my plans." Or, "If you think I'm making a mistake, talk to me about it, don't circumvent my authority with my children." Or, "I'll be glad to hear your opinions if you explain them to me directly." Or, "So please take it easy, and let me be the parent."
This approach will help establish your grown-up-itude, while being respectful to both your mother and yourself. Obviously, you can combine the steps above into a more natural flow, but I've separated them for clarity. It might also help to practice the steps in front of a mirror until they feel like "you."
Learn more about this approach by googling "Non-Violent Communication." Very useful in all sorts of situations. Good luck. As a daughter, a mom, and a granny, I know you can do this.