How do I successfully ask for a replacement or refund?

I recently bought a cute little t-shirt for my 18 month old daughter that says "What happens at Nanna's stays at Nanna's"...

Anyway, it got a few marks on it from her lunch the other day so I read the care instructions to see if it could be soaked prior to washing. The care instructions said nothing about soaking (just don't tumble dry) so I went ahead & popped it into a warm soak with some Napi-San. When I took it out I found that the transfer had basically melted.

I want to take the shirt back to the store & I would like for them to either refund me my money or replace the shirt but I always seem to be too shy to ask for what I want...are there any suggestions for how I can ask for what I want without coming across as a bitch?????

Thanks!