My 2 year old ate 7 or 8 3 mg tabs of melatonin. should i be concerened?

Without any other: information to go on, I can't answer that. I strongly recommend that you call the American Association of Poison Control Center's Hotline at 800-222-1222. They will ask you specific questions & make a recommendation. In any case, all of the members of your household need to keep their medication securely away from children at all times.

Monitor closely: It’s likely that your child will experience symptoms such as extreme drowsiness, stomach upset, and headache. These shouldn’t last more than a few hours, which is how long it takes melatonin to clear out of the body. In children, doses of 3 to 80 mg have been tolerated (you estimate your child took up to 24 mg). If you become concerned for any reason, call Poison Control at 800.222.1222.