Please don't fret about this. It is a big deal that you two fell, of course it is. But you didn't throw him to the ground or purpously try to hurt him. He's a little boy and he's going to get hurt. A bruise is no big deal... wait until the first stitches or the first broken bone... these things happen and you're not a bad person because it happened. Because you clutched onto him you potentially avoided him having a bad head injury so you could have saved his life.... a cheek bruise, no matter how ugly it may be now, is a small price to pay for the awesome job you did keeping him safe. He thanks you for it!!! Nice job with the quick thinking.