Honestly? I would stop taking offence, and take a step back and in a neutral way evaluate if their behaviour could be considered rude. If the answer in all honesty is possibly, I would address this quietly and effectively.
There is a huge range of what is considered good or bad manners. You are obliged to raise children that know the difference, otherwise they will grow up not knowing how to be polite. This will be the biggest possible disadvantage they could possibly hope to acquire professionally, socially and in their every day life. It will blight every part of their lives, they may not even realise over the years, and grow to have a big chip on their shoulders not understanding why the world gives them the cold shoulder.
Children need to have good manners, understand social norms, be polite and being ‘cheeky’ certainly does not make them the exception, quite the reverse, if you are cheeky you need to be doubly aware of the limits, and be mindful not to offend others.
You are doing them no favours springing to their defence op. Evaluate calmly and carefully from all points of view, and decide if there is any merit in your friends criticism. I guarantee you she did not find it easy to tell you.