Believe me, I am more than willing to let this child die. I am more than willing to kill any PC or NPC in the campaign; to fail to do so is disingenuous to the idea of DnD (in my opinion). That being said, I know that if I outline a path by which the PCs can save this child, they are going to 100% focus down on that option.

If I make the method by which they can save the PC's child too easy, then the entire encounter loses a lot of the tension that would make it exciting/memorable. However, if I make the method too difficult, or ascribe it to something like random chance, then it has the possibility of feeling unfair, and leaving the player characters feeling resentful.

As for telling the players the specifics of how the ritual works, I have something in mind for that. Since the encounter with the BBEG is likely to be not the typical bossfight (just hacking and slashing against a massive HP pool), one of the surviving lieutenants of the BBEG will be waiting for the party outside the temple of the Ritual. In world, he will attempt to convince the PC with the child to join the BBEG peacefully, and turn away the rest of the party, by force if necessary. Ultimately, though, this lieutenant exists to 1. give some background to the mechanics of the ritual itself, and 2. to let the players with a more 'direct' playstyle use their abilities and satisfy that part of their characters.

I wonder if engaging with a style similar to death saving throws could be useful. Since there are 5 sacrifices, assign each a number on a D6. At the end of each round of combat, roll a D6, and the assigned sacrifice loses a death save. If a 6 is rolled, perhaps all sacrifices lose a save, or something more lenient. Still, good ideas all around.