How can one answer children's questions like "Where did God come from?" in a way that suits their age (seven or eight years old)?

Reading the Qur'an to children is a great act of obedience. Explaining some of its meanings is permissible if you rely on reliable books of tafsir (definition) or trustworthy scholars. However, interpreting it based solely on personal opinion, without evidence, is not permissible. Answering their questions about Allah Almighty in a manner appropriate to their understanding is commendable. They should also be taught the dhikr for sleep.