They had plenty in stock on Saturday and were constantly topping up the shelves, I imagine you will be fine. Just check the DBS logo on the back to make sure its not peeling off like mine!
There seemed to be plenty of junior sizes in at lunch time yesterday : Extra small Junior (probably for babies), Small Junior (perhaps toddlers to 3 years), Medium (guess 4-6), Large juniors (guess 7-10) and extra large junior(11 upwards). I can only vouch 100% for extra large junior, it fits my 11 year old grandson and probably will still fit him at the end of the season.
There were sponsors add-ons on the front and back of the shirts
I hope that this helps