Just jump in and know that all of us quilters have made tons of mistakes. Take a deep breath and enjoy the process. It's supposed to be fun. When you finish you will be so proud of yourself. The grandies will just have to learn not to bother it. If they're old enough you might give them something to do themselves. You could precut some blocks on muslin and let them draw pictures with crayons and once it's pressed, it's permanent. Use an embroidery hoop to keep it taut. A friend of mine made pot holders out of her grandies' pictures.