Grandchildren once where called "abiatici" but the term didn't develop within the Italian language so it developed only "nipote" to say both "grandchildren" and "nephew".

I don't know the reason,sometimes you could say: ' nipote di nonno ' or 'nipote di zio' Both expressions are clumsy, but have overcome the ambiguity in 'nipote'.

Ciao Vitória, this is a very good question😊 According to Accademia della Crusca (it studies the Italian language and answer questions about Italian), the fact that in Italian there is only the world 'nipote' probably comes from the medieval family organization, where grandparents and uncles had the same rights and duties over nephews and grandchildren (and vice versa). Actually there is a world in Italian that means only 'grandchild': it is "abiatico", but NOBODY uses it. If you like to deepen this question, you can read what Accademia della Crusca said about it: https://accademiadellacrusca.it/it/consulenza/c%C3%A8-nipote-e-nipote/1148