How to teach your child about original sin is spelled out in Ephesians 6:4 KJV:
And, ye fathers, provoke not your children to wrath: but bring them up in the nurture and admonition of the Lord,
This stanza of this commandment states that children should not be caused pain or upset in order to know that they are born in sin. The Greek root word translated "provoke...to wrath" is παροργίζο (Latin: parorgizo) and refers here to offenses or damages, namely the slightest of personal offense perceived by a child, stemming from entitlement. Children are usually punished ultimately because parents see them as sinners. However, not only is punishment banned under God's Law, but so is any offense as perceived by a child. Avoiding offense in children is the same as avoiding offense in other adults. Just like hurt feelings are inevitable in relations between adults, hurt feelings are inevitable in children when dealing with them. One thing that hurts children's feelings is being punished or shamed as a sinner, including being called a sinner. Whenever you hurt your child's feelings, you need to be willing to give a meaningful apology to your child, lest it officially be an offense. If you ever call your child a sinner out of anger or haste, or else punish them for sin, you definitely need to give a formal apology for hurting your child's feelings with your anger, such as "I apologize for losing my temper with you", and then commit never to your cool as a parent ever again, as any parent anger directed towards a child is entitlement, and was seen as entitlement in the Early Church. This stanza of this commandment cross-references the Eighth and the Tenth Commandments, with the Apostle Paul here convicting a group of Greek Christian parents in the parish at Ephesus who brought into the church their pagan custom of patrias potestas, which is a Latin phrase roughly translating to "power to the parent", namely the power to impose punitive sanctions on children, such as spanking or other forms of punishment of children. However, this stanza of this commandment was ultimately received as prohibiting offense as perceived by a child. Paul, contrary to popular legend, was anti-spanking, and opposed any and all punitive parenting in his secular writings. Attachment parenting was banned under Roman law, but the Early Christians did it anyway, obeying God over men.
You don't have to use punitive measures in order to teach a child about original sin. The Greek root word translated "admonition" is νουθεσία (Latin: nouthesia) and is more aptly translated as "instruction" in more modern versions of the Bible. Paul here was referring to religious instruction, of a specific type. Starting at age 6, fathers left out a Bible for children to discover and explore. Whenever children were caught in the act of studying the Bible, they were lavishly praised and encouraged to keep on studying. Fathers then said something along the lines of "you can ask me anything about that book". From there, religious instruction came in the form of question and answer, with children being curious about Scripture, and with fathers giving pointers on the context. At some point, the topic of original sin is bound to come up. In most cases, children ask parents "Why did Jesus have to die?". The answer you should give is "We are all sinners, and so we need a Savior to save us from sin".
Sin shaming is the core understanding of most child abuse in this country. Most parent want their children to know that they were born in sin. The idea is not to say "you are a sinner". Instead, children should hear from their parents "we are all sinners as human beings", and only in the most reassuring of ways.
Human nature is not something to idealize. We as humans are glorified apes, meaning God saved mankind from a peacekeeping nature. The human genome is inherently flawed. One flaw of the human genome is the capacity of parents to lose their cool with their children. Most American parents learned to be angry at their children from their parents. Most Jewish parents never get angry with their children, with Christianity being an offshoot of that Hebraic example. Maybe the human genome made sense in the context of peacekeeping, but we were saved from that fallen nature by God.
The depraved and entitled parents who punish or shame their children for their sins will not inherit the Kingdom of God! Let them be cast forever into the lake of fire and burning sulfur, which is the second death prepared for Satan and his accomplices! Let them descend into the abyss which is the ever-burning Hell of fire and torment, suffering God's Wrath day and night forever and ever! Repent, for the Kingdom of Heaven is at hand!