How about the Italian word nipote? Nipote can mean nephew, niece, grandson and grandaughter. It's also singular and plural.
So there is really not a word for just grandson or granddaughter or nephew or niece. Nipote mean all four of them.
nipote noun, masculine
(di zii) nephew;
(di nonni) grandson, grandchild;
nipoti pl (collettivo) grandchildren, nephews and nieces
noun, feminine
(di zii) niece;
(di nonni) granddaughter, grandchild
nipote=grandchild - granddaughter - grandson - nephew - niece
In Spanish per esempio you have a word for each one of these words.

granddaughter=nieta, grandson=nieto, nephew=sobrino,
niece=sobrina - plural=nietas, nietos, sobrinos, sobrinas