I think you are supposed to book a lesson (usually 2 hours long, and comes with aqua shoes, shirts, and a board) with a place like Goofyfoot or Maui Wave Riders. It will cost quite a bit more than $30/hour.
However, with 4 kids, you can have your own private lesson, so one instructor and just your 4 kids.
Also, beware, if the two 6-year-olds can't really swimming, little big waves will knock them out of the board very quickly, and if they suck in salt water, then it could be all over too soon. This happened to my then 5 year old. That 2 hours lesson was over in about 10 minutes, with big crying.
My two kids had a semi private lesson with just the two of them with an instructor at age 8 and 10. They loved it. I was in the water the whole time help catching them if they fall, and help pushing them back to their instructor. So, with kids that young, it might require some parental dedication. Good exercise.