No More Trolley.

Nelson refused to sit inside the trolley this few months.He likes to walk like an adult or take his own trolley. We often shop at Tesco lately because small trolley are available for kids.Sometimes, I was scared he will get lost. Nelson can know pronounce his name. But, if you ask "What is your name?"
He answered :"Nelson Niu Niu."

I do don't understand where the "niu niu" words come from?