Thank you so much for the kind words about the game. My initial instinct was to add the higher value cards as well, but I found that I did not particularly like them. Grabbing a second deck of cards and adding more 1's, 2's, 3's, and 4's to create the 5x5 or 6x6 grid felt much more balanced. But hey, throwing a couple really numbers out there does add a nice bit of chaos to the game - especially if they are still open during the end of the game, and someone gets trapped on one.
I hope you and your sons continue to have a blast!