Yep, they're safe. The alcohol is ethyl alcohol, the drinking kind.
You may be disappointed by the flavor strength though; the usual Schilling-type food flavorings aren't very concentrated and can significantly dilute your
juice if you add enough to make a difference in taste. If you do decide to use them, you might do better by letting most of the alcohol evaporate off first. Just pour it into a bowl and set it aside for a few days.
Look for Lorann flavorings in a store with lots of baking and candy-making supplies. They're more concentrated and will work better.
Also, be aware that it takes a day or two for the added flavoring to fully combine with the
juice and result in a stable taste.