You're in luck! I'm currently doing my PhD on Plutarch and I can tell you that he does mention child sacrifice in Carthage (as /u/iHistorian said, Carthage was a Phoenician colony) in his De Superstitione, chapter 13 (On Superstition).

No, but with full knowledge and understanding they themselves offered up their own children, and those who had no children would buy little ones from poor people and cut their throats as if they were so many lambs or young birds; meanwhile the mother stood by without a tear or moan; but should she utter a single moan or let fall a single tear, she had to forfeit the money, and her child was sacrificed nevertheless; and the whole area before the statue was filled with a loud noise of flutes and drums took the cries of wailing should not reach the ears of the people.

In another passage in the Regum et imperatorum apophthegmata (Sayings of Kings and Commanders) 19.1 Plutarch says that the tyrant Gelon ended the practice in Syracuse at least:

Gelon, the despot, after vanquishing the Carthaginians off Himera, forced them, when he made peace with them, to include in the treaty an agreement to stop sacrificing their children to Cronus,

It might be worth checking out whether or not Cronus had any relation to Molech or other Phoenician deities.

These may or may not be what you're looking for since Carthage was a Phoenician colony and might have drifted from Phoenician norms, but I honestly can't say for sure. Plutarch can also be kind of a shady source for historical commentary since he mostly dabbled in morality, philosophy, and biography so it's worth taking what he says with a grain of salt. Most of his biographies fudge details in one way or another to espouse moral lessons rather than historical accuracy.

Not to mention that he wrote in the 1st and 2nd century CE so he's not exactly contemporary, but there is evidence that he had often done his research and had plenty of resources available to him. Good luck!

Bibliotheca historica by Diodorus Siculus may have what you want. An excerpt from Book XX (chapter 14, verse 5), which describes the response of the Carthaginians (Carthage is a Phoenician colony) during their war with Agathocles of Syracuse, reads:

When they had given thought to these things and saw their enemy encamped before their walls, they were filled with superstitious dread, for they believed that they had neglected the honours of the gods that had been established by their fathers. In their zeal to make amends for their omission, they selected two hundred of the noblest children and sacrificed them publicly; and others who were under suspicion sacrificed themselves voluntarily, in number not less than three hundred.

There are also textual references in the Old Testament (OT) regarding child sacrifice to Molekh (Moloch/Molech), who is often presented as a widely venerated god in the Levant (including Phoenicia) according to many modern theologists [EN1], although an actual proof that "Molekh" is just another name for any other well-known Levantine god has proven elusive because there is little evidence to work with [EN2]. The OT described Molekh as an Ammonite god, but very little is known about the religious cults of the Ammonites.

Leviticus 18:21 (RSV):

You shall not give any of your children to devote them by fire to Molech, and so profane the name of your God: I am the Lord

2 Kings 23:10 (RSV):

And he defiled To′pheth, which is in the valley of the sons of Hinnom, that no one might burn his son or his daughter as an offering to Molech.

End Notes

[EN1] I've come across many articles and webpages by theologians (and even historians) that generalize Molech as a god venerated in Phoenicia. A simple online search will produce a lot of such result.

[EN2] Klaas Smelik (1995). Moloch, Molekh or molk‐sacrifice? A reassessment of the evidence concerning the Hebrew term Molekh. (Peer-reviewed journal). Scandinavian Journal of the Old Testament: Vol 9, issue 1.