Chaos can be fun, but order makes the garden pleasant to be around. We may have planted tall plants in front and small plants behind where they disappeared from sight. Height and width of mature plants need to be considered for good garden design. If you buy your plants and seeds, tags or packets often will state how big the plants get at maturity and how closely to plant the seedlings. Use this information to help you and your child decide where the plants should go.
Big plants go to the rear of the bed generally and the smaller ones go closer to the front.
This insures that the flowers can be seen. There are exceptions to the rule, but the basic principle works. Some plants have big wide leaves and some have spiked ones. Some plants have crinkled leaves and some are smooth. Some flowers are trumpet-like and some are more full.