Turtles can eat many different types of foods, including certain fruits and vegetables. Grapes are one of the foods that turtles can eat with no real problems. Although you will find that some turtles do not like the taste of grapes while others do, they will not affect them adversely in any way.
If you do decide to feed your turtles grapes, you will need to remember to cut them up small enough to where they will not have any problems digesting them. By taking the time to wash off and cut the grapes you feed them, you will ensure that they can enjoy them safely.