Complex sentence: She realizes that she had inadvertently caused Simón's death (she had accidentally blocked the hidden door to the basement where he was playing, trapping him inside), and that the unexplained noises she had heard were Simón trying to get out.
Split into simpler sentences: She realizes that she had inadvertently caused Simón's death by accidentally blocking the hidden door to the basement where he was playing, trapping him inside. The unexplained noises she had heard were Simón trying to get out.