In reply to Rob Parsons:
Tryfan Bach can indeed be climbed as a single pitch (with easy access to/from the top), but also can be split easily into 2 or 3 decent length pitches, which would probably be more appropriate for age/grade. Most of the routes are vertical cracks straight up a slab. It would be an ideal spot for a first climb with a mountain feel, but doesn't actually go anywhere near the top of a mountain, so you'd soon be looking for other places.
Few of the bigger mountain climbing routes have nice clean vertical pitches though, so communication and coaching up awkward steps would be more difficult. And many of the diff's are 6-8 pitches or more, and with shorter days that might risk getting benighted.
Scrambling might well be better at first - mountain feel and climbing short steps (with or without ropes) but without having to have set pitches/stances
The other option would be to start looking at sea cliffs - Symphony Crack at Rhoscolyn is a short easy top-ropable climb, but with a very different feel to gritstone! Several other areas nearby also have easy and top-ropable climbing - e.g. Porth saint and Porth y garan.