I was reading the book "Ghosts of Cannae", about the Second Punic War, and when it was talking about Carthage's society, it mentioned that it was believed Carthaginians practiced ritual sacrifice of infants. From what I understood, the evidence was a cache of ancient jars containing the burned remains. Is it sure if these were actually sacrifices, or could it have been some kind of ancient cremation?
The burial grounds with the cremated children are called tophets. There's a large one in Carthage, but other smaller ones have been found in other Carthaginian colonies. The bones are of children ranging in age from fetal to ~4 years old.
The question, as you say, is whether this is a practice of cremating children who had died young and burying them in a collective graveyard, or of actual child sacrifice. In any pre-modern society the rate of child mortality would have been high, so it is possible the children died naturally. But the Greek sources are fairly adamant that the Carthaginians sacrificed children (those who have the mind can find particularly grisly examples at Diodorus 20.14.4-7 and Plutarch Moralia 171C-D). Furthermore, Carthaginian dedicatory inscriptions strongly suggest the cremations were of a sacrificial nature:
"It was to Lady Tanit Face of Baal and to Baal Hammon that Bomilcar son of Hanno, grandsom of Milkiathon, vowed this son of his own flesh. Bless him you!" (CIS i.5507)
So all in all it seems likely to me that they did perform child sacrifice, though the topic is still a matter of open debate in scholarly circles.
There's a very nice and succinct discussion of the matter in R. Miles' Carthage Must be Destroyed (from which I borrowed the Carthaginian inscription above). He provides more of the scholarly bibliography on the topic if you're interested.