what is the minimum ios supported for square?

Can someone please help me with a few questions. I am the representative from the Terryville/Plymouth Little League. We are researching the best and inexpensive way to try and offer the ability to pay with a credit card at our concession stand this year. We are a non profit organization and ran solely on volunteers. I was just looking at using "Square". Does any one know what the minimum ios for an ipad to use this credit card? Thank you for your help!!!!