We were there a couple of weeks ago, only stayed for two nights and wish it were longer! We visited Tenby and Barrafundle. Barrafundle involved a 25 min drive, walk up some stony steps, walking along the cliff (near enough to the edge to get an amazing view but far enough to feel totally safe.) then down some steps. It's a bit of a trek but it's worth it. That said, I'd choose Tenby over Barrafundle, although Tenby is probably busier.
Our 3 year old stayed in the pram for most of it but it would have been ok (safety wise) if he wanted to walk it.