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."