My 22 month old suddenly developed dark under eye circles we went to the park 3 days ago.it was a hot day,he played with a dog, and picked flowers. we've done the exact same outdoor activity be4 but he never had dark eye circles then.will it go away?

This is probably due to melanin procuction and related to your mutual ethnic skin type. I instinctively tried to shade our kids from over exposure. But occasiond do arise when the best thng to do is use an ultra violet light filtering skin cream when they are going to be out in the sun. However, a little sun exposure is good in stimulates Vitamin "D" production. It is all about balance.

Hard to know what this is based upon your description. Could be mild trauma or allergy. I doubt it's anything serious or whether the outdoor activity had anything to do with it. If it doesn't resolve in a week, you should see someone to rule out a more serious cause. There are some very rare neoplasms that can cause this too. If it goes away, I wouldn't worry about it.