In reply to tjekel:
There are certainly areas at Margalef -- but its a bit of a trek from Tarragona -- depends on how much you like climbing on pocketed conglomerate.
I also know of some single pitch bolted granite roadside crags that are not far from Poblet - and there's good laybys and some flattish areas down by the stream - however I believe all climbing was been banned in the area at least 10 years ago as the area is what we would know in the UK as an SSSI. It's getting on for 15 years since I climbed there.
From what I remember there's not a lot in the Prades mountains that I would class as particularly small child friendly -- I had a struggle persuading my wife to accompany me to most of the places I visited!