If it were me, I would go ahead and make them now...no stress, no rush, no time frame. You don't have to give it to them now...wait till graduation if you choose but they will be done. With that in mind, I would choose a clean, symmetrical pattern in neutral colors for the boys and a two color only pattern...white and their favorite color for the girls because they probably already know their favorite one now and chances are it probably will still be their favorite down the road. If I wanted it to be something they held on to into college and marriage I would not choose school colors or sports logos. Like I said.....just if it were me........