If "offside" is called in your age bracket, you can teach this simple version: You are not offside if you are doing any of the following:
The penalty for Offside is that an Indirect Free Kick is awarded to the opposing team to be taken from the place where the offside occurred.
When my forwards "push up" without the ball, I tell them to stay 2 steps behind the Last Defender (not counting the goalkeeper) so they are less likely to be caught offside or to be accidentally called offside. (See "Played" & "Offside Rule, Detailed").
See prices for our iron on Motivational Soccer Patches