The Mexican Spanish dub is notable for not using celebrities in the dub and relying entirely on professional voice actors, and it's also considered by many critics in Mexico as one of the best Mexican dubs on history.
Young Simba (his singing voice) is voiced by the Afro-Mexican singer Kalimba Marichal, years before he became a singer and when he was a voice actor instead. He also voiced young Keiichi Morisato and Chicken Little.
Mufasa was voiced by the late Constantino Romero, one of Spain's most beloved voice actors of all time, as well as a successful TV host. To summarize, he voiced Darth Vader, the T-100, Roy Batty and disturbingly enough, Judge Frollo. He was also the regular voice actor of Clint Eastwood. Romero said pretty often that Mufasa was one of his absolute favorite roles.
Young Nala, on the other hand, was voiced by Graciela Molina, one of Spain's most recognizable voice actress thanks to her unique beautiful voice. She played Ariel, and she's the regular voice actress of Kirsten Dunst, Amy Adams and Christina Ricci, among others. She's the One-Scene Wonder of the movie for Spaniard voice acting enthusiasts.
In the second movie, young Kiara was voiced by Michelle Jenner (Miguel Ángel Jenner's daughter) who years later would played Hermione and become a famous live-action actress in Spain.
Young Kovu was voiced by Masumi Mutsuda (a Spaniard voice actor of Japanese descendent) who later played young Anakin Skywalker.
The case of adult Kiara and adult Kovu is particularly hilarious, since their voice actors are actually siblings! Albeit since voice actors' identities in Spain are pretty unknown for the most part, even more back when the movie was released, no one was squicked.