The campaigners have been working with their teachers Mrs Pearson, Mrs James and Mrs Tidbury to get the word recognised.
Ten-year-old Kirsty explained: "It's a good word because 'nieces and nephews' has five syllables, and 'niblings' only has two."
She said the kids were "really confident" that their campaign would succeed.
Jamie, 11, said: "All my friends and family are saying the word. It's getting bigger and bigger.
"It would be amazing to see it in the dictionary."