Grades are determined by the age of players as at 1 January each year. The name of the grade refers to the age players will turn in the coming year.
For example a player who is 8 on the 1st January will be in 9th Grade.
So to find out which grade you or you children are, the simple rule of thumb is to use the age you will be as at the end of this year. If you are going to be 9 you will be in 9th grade; if you are going to be 7 you will be in 7th grade and so on.