Why did Adam and Eve never have any children before transgressing with an apple from The Tree of knowledge of Good and Evil? And if they had, why is there no mention of it in the Torah? Eve was punished by pain at childbirth, does it mean that she had previous childbirth experience without pain?

Cain and Abel were born before Adam and Eve sinned and their births were immediate to their conceptions, without the pains of childbirth (Sanhedrin 38,2). The Torah does not mention explicitly when they were born. But where did you read that the fruit of the Tree of Knowledge of Good and Evil was an apple? Don’t fall prey to Christian imagery.