
Personally I have never used agents (although JK Rowling is a good example of somebody who DOES! :) ) as the downside of course is that a) the work has to be accepted not once by twice -first by the agent, then by a publisher. (The upside being if an agent has accepted it, you can be more sure that it IS worthwhile persuing!) and b) you end up not getting all the money for yourself. :) I'm not actually sure it is much quicker to use an agent as after all they will have other clients and will have to approach publishers and they publisher will still want to read part of the manuscript (they do not usually want the whole thing, just enough to give them an idea of what it is like).
There is a book called "The Writers and Artists yearbook", published every year, which lists all UK publishers and also agents and it does give details such as what type of book they publish, how to approach each one etc. Amazon usually has it.
Be aware that getting something published can take a very long time -especially if it is intended for children, as there are so many people submitting manuscripts and only a small amount gets accepted. It took me 18 months to get my first book accepted, but often it pays not to give up -again lots of famous writers were initially turned down.
Good luck to your mum!