Well it's really up to yourself. Watch some Youtube videos of both Cashel and Blarney and ask your two boys which they would prefer. It will get them involved and more interested. As for lunch there's no shortage of places near either.
You may find that your kids prefer Cahir Castle to the Rock of Cashel. The Rock of Cashel is more historically significant, but Cahir Castle is a more complete and coherent castle (the fortifications of the Rock of Cashel were largely replaced with ecclesiastical buildingd in the 12th century).
In Cahir, there's a good cafe just across the road from the castle, next to the river, but the name escapes me at the moment.