Milkshakes advertised, but not for everyone

I really love Spur. But can someone please help me understand why I can't order a kiddies Milkshake , since it is advertised for the whole world to see, but when I ask about getting one , I am told that I cannot get it because I am not a child ? But my child can eat my food right ? I don't understand. If I wanted a kiddies meal at Burger King that's fine, but this is a first for me. I just don't get it .