Bart, my son-in-law, is gifted with a green thumb. But until the family moved to Colorado last summer and acquired a big backyard, his nurturing went toward house plants. Now he has a vegetable garden, where sons Henry, 2 1/2, and Jack, 1 plus a tad, like to watch things grow. Most promising at the moment are tomatoes. They're taking shape, and some are bound to end up in this simple relish, which my daughter, Angela, likes in toasted brie cheese sandwiches. Bart prefers it with pork chops. The vote's still out with the wee laddies.