For all we know, Dudu might not even be a major character. Also, maybe Dinklage doesn't want to play the character.
Also to be perfectly honest, Dinklage isn't even really all that inspired for Dudu. The only reason people think that way because he's a dwarf height character, and Dinklage played a character like that on a dark fantasy series already. Not saying he could never play Dudu, but there's no telling what plans they even have for the character.
The main ones you have to look at are Geralt, Yennefer, Ciri, Triss Merigold, Emhyr/Duny, Dandelion, and probably people like Vesemir, Eskel, Lambert, Regis, the Lodge of Sorceresses, maybe Vilgefortz as a seasonal villain.
They could cast Ciri as a little girl with the hopes she will grow up with the show like the Stark girls on Game of Thrones. So they might cast like a 12 year old as Ciri.