Probably the last place you would think of growing grass is in a sandbox. However, children can come up with the wildest ideas. They may want to maintain a "lawn" like their parents do---only on a smaller scale. They may view the sandbox as their own personal space to take on this task. Although grass doesn't typically grow on sand, it is possible to grow grass in a sandbox with a few preparations and continued care.
Apply a soil conditioner such as peat moss to the sand and mix in thoroughly. This will condition the soil and help retain moisture.
Apply fertilizer at a rate of 2 to 3 lbs per 100 square feet. For small areas, it is easiest to evenly sprinkle the fertilizer by hand.
Spread grass seed at a rate of 4 to 5 lbs. per 1,000 square feet over the area you wish to seed in the sandbox. Once again, for small areas it will be easier to evenly sprinkle the seed by hand.
Rake the area gently to encourage seed-to-soil contact. You'll want the seed to be buried approximately 1/8 inch into the soil.
Water the seeded area lightly to moisten the soil. Keep the seeded area moist until germination occurs.
Water two to three times per week for the first month and then move to deep irrigation of onece to twice per week. Apply 1 inch of water per week. Use a pan to measure the amount applied.
Mow the grass when it reaches about 3 inches tall. Because the grass is in a sandbox, you'll probably be mowing unconventionally with scissors or another shearing device.