I agree that you should not distract from your wonderful octagons. This is myidea of a perfect kid quilt, lively with lots to talk about. I think you might consider making it easy on yourself in the quilting. The center section could be done easily by treating each octagon into a square, that is, doing a simple gridwork in black, then doing the same with a grey for the border. I'd probably meander the written area. Whatever you decide to do, it will be a fine quilt for the purpose, and certain to be precious to your DGS. He will be lucky to have it.