I have struggled with this for some time and after several failed attempts, I am asking you Mommas for your insight. My 8yo boys speak poorly to and about younger kids, particularly girls, including siblings of their friends (even though they consider some younger childrenas well as some girls to be their friends). My reasoning "you were that age not too long ago", or "Grandma is a girl, too", or "you are an important role model for younger ones to learn from", or "how would you feel if your older friend spoke poorly to you like this", are showing no results. I am not trying to shame them into accepting others either, and yet, I am stuck. I am very frustrated and never imagined to be the mom whose children behave this way (since this wouldn't have even come into our, nor our friends' minds growing up). Where did I go wrong? How do I reach their hearts?

@Rita Guihan 29 , even if this isn't something you've personally experienced, I think what you're seeing in your boys is very normal. They are figuring out who they are and how that is different from or the same as others around them. They may be dealing with insecurities and handling those insecurities by thinking of themselves as better than others. I think we all have a tendency to do that sometimes - even to distance ourselves morally from our children and to think of ourselves as morally superior to them when really, we're more alike than we are different.

I think the first step in being able to address this problem from a gracious perspective is to see how you can identify with them. We don't all have exactly the same struggles, but we all have the same sorts of struggles. Whatever someone else is facing, we ourselves have that same temptation in some variety or other. I know you're not seeing that right now because your boys' behavior feels foreign and odd to you.

I'd encourage you to "zoom out". Think of your boys' struggles in more broad terms - as broad as you need to go until you can see some parallels between their struggles and yours. Who do you not like being around? Who are you tempted to feel superior towards? Can you go to your boys and say, "I know what it's like to struggle with this. Here's a way mama struggles with this. This is what I do or what I say to myself when I have that struggle"?

The biggest difference between kids and grown-ups is that kids say what they're actually thinking, while grown ups know how to play the game in more socially acceptable ways. Your boys aren't so terrible. They're just more transparent!

First of all remember that we cannot force a heart change no matter how much we want to. We can teach, correct, and even discipline when behaviors are unacceptable, but we cannot change a heart. Changing hearts is God’s work. Our job is to share the scriptures and continue to teach them appropriate words and responses. 
As far as practical how to stop behaviors, you might consider removing opportunities to play with others if he is disrespectful to younger kids involved. In our home, I try to require respect between the kids. If they are not getting along with their siblings, then no outside friends for a period of time. If we have others over, I expect their friends to treat younger (sometimes pesky) siblings with patience and kindness or they are asked to leave. 
Having our children persist in an unlovely behavior is very humbling. Remember to pray with and for your son about his attitude. Please understand I speak from in the middle of the daily struggle, not as one who has it all figured out.

Rita, try not to be too hard on yourself in "going wrong". It sounds to me like you are saying some great things and your awareness of the behaviour is immeasurable! But like Laney said, the reaching the heart will happen through Christ. Staying vigilant and loving them and knowing you have nothing to be ashamed of either is difficult work. Prayers that you will see God's work in them sooner than later ❤