A bartender recounts a tale that implies Ciri in the first game, and she's both implied and (I think) name dropped in Assassins of Kings during one of Geralt's memory flashbacks. The one where he remembers who saved him and Yennefer.
As much as I love CDPR's writing both the first and second game occasionally stumble by throwing out new information that to folk unfamiliar with the books will go right over their head as it's never referenced again. Geralt regaining memories of tracking the hunt and having Letho assist him are logical and easy to understand, but suddenly going "OH AND THEN CIRI SAVED US" makes absolutely no sense to those who've only played the games. CDPR have, at times, stumbled a bit with the balance of assumed and presented knowledge given the two audiences involved.
But Ciri's importance is never stated in either of the games, and Yennefer only becomes relevant by memory in the second. I feel what happened is that CDPR started their franchise with as much of a clean slate as possible: Witcher universe game set post-books, starring the lead protagonist, memory wiped to disconnected from book events, and the cast mostly composed by supporting characters. Yennefer and especially Ciri are the most important characters in the books and they're nowhere to be found until Wild Hunt. The first game could stand alone for most part as it's own adventure with or without book knowledge. Assassins of Kings starts getting heavier into the book stuff, mostly when it comes to Yennefer, and even then that's just a backdrop to Geralt's memory rather than anything direct. Wild Hunt on the other hand is a full sequel to both the game arc and the book arc.