If you mean it to be something like "Number one grandchild" then it is 一番の孫
"Number one grandson" in Japanese would be "ichiban no mago." It's pronounced: ee-chee-bon no mah-go.
'Ichiban.'
Ichiban.
"Ichiban kakkouii"
"Ichiban chiisai."
"Ichiban" or "Hitotsume." Ichiban can also mean "best."
Ichiban hayai
ichiban sukina aji
Japan is the best = Nihon-wa ichiban. Japanese language is the best = Nihongo-wa ichiban.
neruno ga ichiban
いちばん (ichiban)
'Ichiban suki na'