Some of the later steps may be beyond them, depending on the child's age, but Charles Petzold's wonderful book Code explains how you could set up a processor fr

Some of the later steps may be beyond them, depending on the child's age, but Charles Petzold's wonderful book Code explains how you could set up a processor from logic gates with only a little handwaving. If they are a little older and interested, NAND 2 Tetris is a good self-paced course.