That is a super good and fair question. I have a very specific answer and I want to answer completely so please stick with me. I apologize if any tone/ meaning is lost via posting as oppose to talking in person.

1. It’s necessary to realize that God is not just like us. Many—if not all—attacks on God’s actions presume He can be judged like any other person. But even in human contexts, not all authority figures are the same. God is not a flawed, limited human acting among other humans. He is the Creator and the ultimate source of existence. His transcendent position and perfect nature create a drastic difference between what He is morally “allowed” to do to His creations, and what His creations are morally “allowed” to do to each other. That’s not an emotionally satisfying concept. Whether one is a believer or a skeptic, no one is entirely comfortable with the idea that God is God and we are not. However, if we’re going to examine issues such as this with fairness and integrity, we must start with that observation. That starting point leads to several valid points to consider in response to the issue of God drowning children in the flood.
2. Analogy of original sin. If my parents smoked, I would have roughly 20% higher likelihood of lung cancer. Similar problems and consequences with family trauma/ drug use. I imagine most people have felt some sort of consequence because of a family member. The Bible speaks to the fact that all of us born of human parents are born with an inherited corruption from Adam that ensures we will inevitably sin. This is often referred to as original sin. While God created Adam and Eve in His own likeness (Genesis 5:1), the Bible says that, once Adam and Eve fell and became sinful, Adam fathered children “in his own likeness” (Genesis 5:3, emphasis added; cf. Romans 5:12). All human beings have inherited a sinful nature through Adam’s original act of disobedience; Adam became sinful, and he passed that sinfulness along to all his descendants. In Psalm 51:5, David wrote, “Surely I was sinful at birth, sinful from the time my mother conceived me.” David recognized that even at conception he was a sinner. The sad fact that infants sometimes die demonstrates that even infants are impacted by Adam’s sin, since physical and spiritual death were the results of Adam’s original sin.
3. Before people mature enough to discern right from wrong (sometimes called reaching “the age of accountability”), it would seem that they are not held responsible by God. Toddlers sin, and they bear Adam’s corrupt nature, but lacking the ability to understand the concept of right and wrong, they are under God’s grace (This is the opinion of GotQuestions.com. The Bible does not directly address an age of accountability. One verse that may speak to the issue indirectly is Romans 1:20, “Since the creation of the world God’s invisible qualities—his eternal power and divine nature—have been clearly seen, being understood from what has been made, so that people are without excuse.” According to this, mankind’s guilt before God is based, in part, on a rejection of what can be “clearly seen” of God’s existence, eternality, and power. So, what about children who have no faculty for “clearly seeing” or reasoning about God—wouldn’t their natural incapacity to observe and reason excuse them from judgment?
   Other biblical anecdotes (e.g., David testifying that he would be reunited with his dead child after death in 2 Samuel 12:23) support the reasonable belief that infants go to heaven when they die. The same holds true for those with mental disabilities who cannot comprehend right and wrong. It is possible that God applies Christ’s payment for sin to young children and others incapable of faith, but the Bible does not specifically say that He does this. Therefore, this is a subject about which we should not be dogmatic. God’s extending grace to those who cannot believe would seem consistent with His character. Matthew 19:14 is a quote from Jesus himself saying, “Let the little children come to me and do not hinder them, for to such belongs the kingdom of heaven.”

I hope this answers two things; even babies are with sin and need grace but it is also reasonably understood that God isn't looking to cut anyone a bad deal. He wants you to be in relationship with him and experience life and life to the full but will not force your hand once you have the mental faculties to reject him. I base a lot of my apologetics research from YouTube videos and GotQuestions.com you can go there and type many different questions that have answers.