Boooo
Set them on or near a heat for a couple of weeks, that should work just fine. Make sure you get not the finest Pine. My Manual suggests #3 Pine...(my instructor advised us NOT to purchase from Lowe's as their stuff is really tough to break - it's better for shelves I guess ^_^)
In the ITA we have staggered board sizes for Juniors. 9 year olds break a 5 inch piece of 1x12 or a yellow re-breakable board, 10 and 11 break a 7 inch or orange re-breakable.
The secret is - really - a swift execution of good technique. 90% of it is mental I think. (how long do you have for prep time?)