Hi Gisgirl24,
Welcome to the WBT community! You are on the right track with the scoreboard. It's true, you don't want to difference to be more than +/- 3 points between the

and

. However, you don't subtract the frownies from the smilies...the 'winner' is the side that has the most.
As far as rewards, you can use each days 'winner' to count towards a bigger prize or you can use small rewards such as more/less homework, more/less recess, etc. I'm not sure what a Destination Imagination team is, so I'm not sure if these will work for you...but hopefully it gives you an idea.
I hope this helps!
Tiffany
WBT Intern, 2011-2012