Fertilizers are fairly safe for children, but they should be applied exactly as the directions suggest. Children should not be allowed on the lawn until the fertilizer has been watered in with a least 1/4 inch of rain. Herbicides and insecticides are far more dangerous to children than fertilizer.
Using organic fertilizer greatly decreases the risk of chemicals from a fertilizer harming a child. Over-application of fertilizer does not increase the effectiveness; it only provides unnecessary exposure to anyone that comes in contact with it. Applying the fertilizer with a licensed pesticide applicator further guarantees proper application, and hiring a licensed fertilizer technician to distribute the fertilizer ensures a safer application.