Chatting with Anthony while he played with a dump truck toy, and trying to expose the idea of Japanese and English as being two separate languages (a concept I'm told that he'll not necessarily truly understand for many more years), I said:

Daddy calls that a “dump truck”, and Mommy calls it a “ダンプ (danpu)”.

He then responded (in English):

Daddy say “dump truck”.
Mommy say “ダンプ”.
Anthony say “ダンプ”.
Only one people say “dump truck”.
Only two people say "ダンプカー (danpu ka- -- longer version meaning the same thing)".