Answer: A really important first step is to make sure the wagon has adequate drainage holes in the bottom. If not, it will fill with water during the first rainstorm! Most wagons are only 4"-6" deep, reducing your choice of plants to annuals, or those perennials with very small root systems. You can use potting soil or a mixture of potting soil and garden soil to fill the wagon, but don't use straight garden soil as your only planting medium. (Garden soil can dry out so completely that it's hard to re-wet.)
Children are always impressed with big flowers in bright colors, but with the shallow planting container you might want to concentrate instead on small plants, perhaps with fragrance rather than large flowers. Wooly thyme would be a good choice, as would chamomile, viola, and rosemary. To add a little height to your garden, plant dwarf marigolds or nasturtiums. All of these plants are child-safe.
Hope your little red wagon is a success!
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