Jeez when you figure out how to deal with second graders let me know because I cannot. The difference between my 3-4-5 year olds and your 7-8-9 year old is quite pronounced, but I can give you some basic pointers that may apply.

1. Model behavior

Act like you want them to act. Say thank you, say please (but don't make instructions sound like an option). 

2. Listen to them actively

When they ask questions or make a comment, rephrase what they say. it lets them know that you're listening to them, and if you repeat it back wrong, they'll let you know and clarify what their saying. 

3. Random things

Long term projects are super fun, letting them have a say in how they learn is always a plus, group works seems like a good idea but IT IS NOT, and respect all modes of learning (make sure you take note of visual, auditory, and kinesthetic learners).