First off, if you want to pursue a career in psychology, you will need to get a BPS Graduate in Psychology diploma (check the bps website) before getting onto some MScs and definitely doctorate programmes in clinical/child psychology. You might be able to get onto a psychology MA given your sociology background, but without any background in psychology, I guess it would be pretty tough to cope with a masters level course in psychology (I've got 2 psychology MScs). I think even if you wanted to do a psychology PhD its pretty much essential that you do the graduate diploma and get some relevant experience.
5) When it comes to applying to doctorate programmes such as clinical (which is what im hoping to do) any masters qualification is given credit in the weighting of applicants; although I suspect that more preference would be given to an MSc given its 'scientific' content