Yes, our children will sin. There is no question of it. I think it was funny when I look back as a young mom and didn't think that my only child did. My older daughter, who was a lot smaller at the time, was a perfect angel---or so I thought. Yes, I know that sounds far-fetched, but I was naive and didn't "see" or want to see it. But the true reality was that she was definitely a sinner needing the grace of God just as much as her unsaved mommy did.
So why is it that we are always surprised when they sin? No, instead we need to know that they will sin. To expect it and be ready and prepared to react in a godly fashion. I have learned over time it is a lot easier if you are prepared. Especially when you have 8 children, as it seems there can be violations all the day through. That is what parents are for, to be an authority figure guiding them in the speedbumps of life.
Children are a lot LESS experienced than us. It hurts me to hear small children getting cursed out by parents who seem to think their child needs to act and perform perfectly like an adult (who sin as well). They are not robots, nor are they our trophy children. They are on loan to us from God for a limited time and we must be faithful with the time that we have.
But we are not here to have PERFECT children. We need to remember that they are not perfect, but are sinners in need of a Savior. They desperately need parents who will lovingly share Jesus and carefully show them the way to Him. As parents we will always struggle doing this since we are not perfect parents but we must strive to do what is right and not grow weary in the process.
They also need parents that have MERCY. We cannot come off as harsh, religious, hypocrites if we are wanting to parent well. We need to exhibit love, patience, kindness, mercy, humility, understanding and be slow to anger. This will win a heart, the opposite will repel it. My father-in-law is a great example of this. He is patient and slow to anger and always has arms outstretched to love on someone. He always shows overflowing love to all his grandchildren (he has 21 of them, and 2 great grandchildren) and they would just absolutely do anything for him.
Think of how God has drawn us to Him. It wasn't through harsh demanding means. Often when I see Him working through sin in my life it was through His kind love and that is what drew me in closer.
Do NOT become discouraged. Children WILL sin and we need to guide them and show them the way. We must remind ourselves that it is all part of God's perfect plan to work all things together for the good of those who love Him according to His purpose (Romans 8:28).
The fruit of this parenting journey is that we are being refined right along with our children and for that we need to praise to Him as we seek to build strong homes to bring Him glory.