In reply to Brian-P:
Tricky. Gogarth doesn't really do diffs/v diffs. I think the easiest regularly climbed routes are around VS - Lighthouse Arete is the classic, but even at that grade choice is limited. HVS/E1 is really where it gets started.
Have you thought about trying north Pembroke instead? Very beautiful cliffs with quite a few easy routes as well as harder stuff. Lovely sandstone slabs.
As for advice: make sure everyone knows exactly what they're doing re. abseiling, exactly what the tide is doing, exactly where you're going ("I think this is probably it" isn't going to be good enough), and that you know exactly what you'll do if you can't do the intended route (e.g. it's too hard, you get lost, it rains, etc, etc). If any of these things are in doubt, don't ab in!
I think there's probably an article on here about sea cliff climbing, have a search.