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 sukina aji
'Ichiban suki na'
いちばん (ichiban)
neruno ga ichiban
You may say "ichiban tsuyoi."
Japan is the best = Nihon-wa ichiban. Japanese language is the best = Nihongo-wa ichiban.