What does "care for" mean, precisely? Is circumcising or baptizing them objectively good, so that they don't burn in hell for all eternity? What about shaping their skull in a more pleasing form? If they have ambiguous but otherwise working genitals, should you do surgery to assign them a clear sex? Or unto more mundane affairs, is it objectively good to give them baby formula instead of mother's milk, or maybe the other way around? Is it objectively good to take them from their parents and care for them yourself if the parents are not caring for them? How do you objectively determine if the parents are caring for them?

That's a bit contradictory, isn't it? If caring for helpless infants is an objective good only for those who inherently understand why that is, then that's a dependence on the observer's understanding and so it is subjective.

There's a world of difference between something being objectively a certain way, and between feeling really strongly some way about something and thinking that everyone else reasonable would feel the same way too. There are things that are encoded into (most of) our very instincts, things we (for the most part) find absolutely common sense, but this doesn't make them objective. I wish language was able to succinctly express these different levels of "being on the same page", but alas I don't believe it does at the moment, and abusing the word "objective" I can't say I love as an alternative.

I agree with you that it is good to care for helpless infants. The fact that this cannot be clearly explained to someone who doesn’t inherently agree indicates that this is not an objective good, though.

The devil’s advocate would probably also ask how it would be objectively good to protect baby Hilter, knowing that protecting his innocent infant life would lead directly to the deaths of millions.

Is caring for a helpless infant objectively good if it is infected with an extremely virulent plague that will undoubtedly kill any human who comes in contact with it, or a human who comes into contact with them, or them, many layers deep? What if that infant has 2 days to live no matter what, but millions of people will die if it's cared for?

Objective good does not exist, context is king.