My ex lives in FL with my kids and I live in Ohio, is there anything I can do if I go down to pick them up for the summer and she refuses to let me take them?

Do you have a visitation order that permits you to have them for the summer? If so where was that order issued (FL, OH, someplace else)? Do you have any indication she will not turn them over?

Not unless there is a custody or visitation order that she is in violation of. If there was never any official determination of custody, you need to petition for one immediately.