Pelagius? He was considered a heretic of the worst kind, as he averred babies are born tabula rasa[I/].
Of course Pelagius was called a heretic. He was contra Augustine. As for babies being born 'tabula rasa' - "a clean slate" - we can agree.
When does a person become responsible for their actions? When does a person become, "a sinner?" At conception? At birth? At some age-of-accountability? Christians have been debating this issue for millennia. If it wasn't for this question there wouldn't have been Anabaptists or modern day Baptists; Amish or Mennonites.
All you have to do is watch infants. You do not have to teach them to lie, steal, hit, &c. That's already built into them via the fall of Adam. However, you do have to teach them to not steal, lie, hit other ppl, &c. Babies are not born sinless at all. As Dr. Voddie Baucham said, "They're vipers with diapers."
Are you suggesting infants lie, steal, or consciously comprehend what it means to violate any of the Ten Commandments?