I was in North Wales this past week. Llandudno was good - you can walk up Great Orme, or pay to take the Tramway. I stayed at a great place - Clontarf Hotel (though it's more an adult type B&B I'd say, don't think they want noisy children, though do have one family room) which is very near West Shore which is the quieter side, yet town is still only a 10 minute walk away.
You could base yourself in Llandudno and then get to other places quite easily by car - such as a walk around the walls of Conwy (there is a children's playground just near the carpark by the hotel/cricket green).
There are many places you could visit, especially if your 5 year old is interested in all things slate related - as North Wales has a lot of slate.
Snowdon can be walked up... or there is a train. LLanberis is the base point where there is the National Slate Museum which my 4 year old nephew liked... think we were there about 1.5-2 hours (Free, though £4 for the carpark - all parking in Llanberis is expensive but is usually all-day parking).
There are several steam train lines in the area - Ffestiniog Railway
There are also Llechwedd Slate Caverns which has two trains... one which goes deep under ground.
There are of course Castles to visit.
The Center for Alternative Technology is further to the south, probably around a 1.5 hour-2 hour drive.