Hi Simon
As a Man City fan I had to be brave and open the site but it looks like a nicely built Zen Cart site and he calls to action are good (wont be buying one by the way).
You say main keyword target in your question and this is often asked here and my first advice is not to focus on a narrow keyword set. I would look at a list of keywords in the 3-4 word range and at least have list of 15-20 word sets and direct your pages in this direction.
The football / soccer shirt space is really busy and I would spread my wings in the keyword space a bit to try and capture some of the other keywords (I was prompted Arsenal Shirts for kids when I typed in a quick Google search)
I would also spend some serious time on your product descriptions and try and make them unique and richer for people and for search. For instance at random I picked
_emulate the skills of star players like Aaron Ramsey, Marouane Chamakh and Thomas Vermaelen. _
and there were at least 4-5 sites with this identical text (and yours wasn't even there). This does not help.
I would also look at making your URLs more search friendly. Zen Cart has some plugins to accomplish this for you and wouldn't hurt either.
http://www.zen-cart.com/index.php?main_page=product_contrib_info&cPath=40_47&products_id=231