Actually on this aspect there is no contradiction, for what Deuteronomy 12:30-312 (cf. Dt. 18:10; Le 18:21; 20:2; Jer 7:31; 32:35) forbids is man making children sacrifices in their ignorance or regardless of their will, which is not the same as a Son choosing to die, and not by self-inflicted mortal means, but by voluntarily allowing men to do what they will.
The captain of a army may determine to rescue men from a POW camp, which will someone to infiltrate its headquarters to kill the commander as well as create a distraction and keep enemy forces focused on him as long as he can, and face certain death in so doing, so that others of his team can launch an attack on the rest and free the POWS.
The one who chooses to be that man does not do so out of compulsion, or ignorance, but freely chooses to lay down his life, knowing what the enemy will do. This is not what the prohibition of child sacrifice is against
The Catholic Church inculcates its adherents to the belief that the Catholic Priest brings JESUS from Heaven to the Catholic Altar, to continue the 'human sacrifice' JESUS finished at Calvary.
And they affirm it is the same but deny that Jesus is still being offered continually as a sacrifice.
And forasmuch as, in this divine sacrifice which is celebrated in the mass, that same Christ is contained and immolated in an unbloody manner, who once offered Himself in a bloody manner on the altar of the cross; the holy Synod teaches, that this sacrifice is truly propritiatory...For the victim is one and the same, the same now offering by the ministry of priests, who then offered Himself on the cross, the manner alone of offering being different. - Trent The Twenty-Second Session, cp. 2; http://history.hanover.edu/texts/trent/ct22.html
1265. What is the Sacrifice of the Mass?...Christ, the eternal High Priest, in an unbloody way offers himself a most acceptable Victim to the eternal Father, as he did upon the Cross.
1269. How does the Mass re-present Calvary? The Mass re-presents Calvary by continuing Christ’s sacrifice of himself to his heavenly Father. In the Mass, no less than on Calvary, Jesus really offers his life to his heavenly Father.
1277. Does the Mass detract from the one, unique Sacrifice of the Cross? The Catechism of the Council of Trent: The Mass in no way detracts from the one, unique Sacrifice of the Cross because the Mass is the same Sacrifice as that of the Cross, to continue on earth until the end of time...The Mass, therefore, no less than the Cross, is expiatory for sins; but now the expiation is experienced by those for whom, on the Cross, the title of God’s mercy had been gained. (John Hardon, The Question and Answer Catholic Catechism (Garden: Image, 1981).
We, therefore, confess that the sacrifice of the Mass is one and the same sacrifice with that of the cross...That the holy sacrifice of the Mass, therefore, is not only a sacrifice of praise and thanksgiving, or a commemoration of the sacrifice of the cross; but also a sacrifice of propitiation, by which God is appeased and rendered propitious.. (The Catechism of the Council of Trent, Published by Command of Pope Pius the Fifth (New York: Christian Press, 1905), pp. 173-175).
You will never get the poster of that ‘apparent contradiction’ to admit any error on his part. IH runs deep with that one.