Hi Stuart!
Thank you so much for your lovely compliment! I think that I speak for all of my fellow 'Moms' when I say that we love nothing more than helping people to plan their holidays to Walt Disney World!
Being from the UK myself, I know exactly what you mean and you will be pleased to hear that there is a selection of clothing available to buy throughout the parks and resorts for your 18 month old child.
The best place to visit for shopping would without doubt be
Downtown Disney. There is a large selection of shops here - my absolute favourite being World of Disney which is the largest Disney Store on Earth! Music to my ears although I'm not sure that my Partner would agree when I am let loose in there!
My son loves wearing his character t-shirts and tops whilst we are visiting and the characters really do make a fuss of you when they see that you are wearing something with their picture on.
I understand that you want to pack light but we have always found a lot of reasonably priced Disney clothing at ASDA and tend to stock up on several items of clothing for my son from there before we set off.
Please come back and ask me another question if I can help you with your planning any further and above all else have a fantastic holiday!