Is melatonin (3mg) safe to take while breastfeeding my 7mth old? I have come down with insomnia, and want to know what is safe to aid sleep

No good data. Melatonin is from the brains of many dead animals, who really knows their health status or if they were infected with some strange virus? Agriculture is far from hygienic. I would avoid this product. The supplements are not regulated in any significant way.......For purity, country of origin, look up the consumer's reports study on echinacea.........
Uknown. Information regarding safety and efficacy in pregnancy and lactation for exogenous melatonin is lacking. In children 6 months to 14 years of age with sleep disorders melatonin 2 to 5 mg has been used safely.
Try something else. The risk to junior would be limited to scrambling the developing sleep cycle. However, there's talk of melatonin suppressing prolactin production by the pitutiary gland which would interfere with lactation. There's little real science here. If poor sleep is interfering with your waking life, it's for you and your physician to weigh benefits and risks.