When the plague of Egypt kills the children of Egypt, that is for atheists evidence of a cruel God. For Jews and Christians it is evidence of the way by which the Jews are to freed from slavery and the House of David is to be founded from which will spring the Messiah. For the atheist God is cruel to kill the children because the atheist believes that killing the children means taking their lives away. But the souls of children are not annihilated when they die. The children live on. We may believe they do not live in hell, where possibly some of their parents may reside. But the subject of limbo is for another thread.
God had through Moses given the Pharaoh abundant opportunities to let his people free. Pharaoh chose not to and sent his soldiers after the Jews to destroy them. The soldiers were destroyed. The atheist will argue again that God was cruel. But actions have consequences. To disobey God is not smart. It is no smarter than to defy our human God-given nature, which the folks at Sodom and Gomorrah also found out.
And that, it seems to me, is what it always comes down to when people complain of the Old Testament stories as tales about a savage God. These complainers never understand that when God has spoken, he should be obeyed. :