Prepare a planting site that gets full sun and has well-draining soil. If your soil is low in organic matter, help the child spread a 2-inch layer of compost over the soil, then work it in at a depth of 6 inches with a shovel or tiller.
Dig a small trench 1/2 inch deep and 2 inches wide in the planting site. Cut a length of radish tape the same length as the trench. If you are planting more than one row, space them 18 inches apart.
Give the radish tape to the child and have him lower it into the trench. Radish tape is a biodegradable strip of fabric with seeds embedded within it, so the child does not have to try to evenly space small radish seeds. Help the child cover the tape with soil.
Fill a watering can with the child and have him water the row until the soil settles. If the child is old enough to handle daily gardening responsibility, have him check the soil each morning to see if it is moist. He needs to water the soil to ensure good radish flavor. You can also use a reminder chart to help him remember to water them daily, if necessary.
Wait three to six weeks, depending on the variety of radish you planted. The child can pull them out of the ground at that point, and then help you wash and prepare them for eating.