It seems that sometimes a societal entity or member of an incapacitated or not-fully-matured individual must make a decision for them, but what is the basis of this moral decision?

what counts as "defense" for a child/incapable person?

The basis is maxisization of individial liberty, that means you make sure they regain their ability to chose if deprived and make sure most if not all choices are reserved when they regain their ability to chose for themself.

Or you leave them for dead

Surely this individual liberty can't come at the expense of that of others, right?

Yes

But also surely, people should be allowed to compete under some conception of a meritocracy. That would result in the meritorious triumphing over not meritorious, at their expense.

However that would be a valid expense, yes?

not everyone can have the "liberty" to be the best

Yes

but some conceptions of competition do not benefit the group, and it is only a tyranny of the strong over the weak by some definition

so then what we really care about is what moral system, what moral principles we should compete under, that benefit the group

did I use any sleight of hand there?

or is it that individual rights and benefits must be balanced with the good of the collective?

what defines that balance?

well, that would be what I suggest as the basis for morality: principles that promote survival and reproduction within the group, that promote power and stability for competition with other groups

and against nature

a balance completely toward individualism would dissolve the group and turn individuals against each other in destructive way

as for a balance skewed completely toward collectivism, it's a bit harder to show why that is not a good idea

I need to introduce a few more things

but the point is... morality is affected by evolutionary forces that act on the survival and reproduction of the group, not necessarily just the individuals

the same morality that we use to make our moral judgements, ostensibly in defense of individual rights, is grounded in a balanced individualist/collectivist morality

and you can call that "individualism" but really this is a misnomer

at least for the principles that allow for moral intervention of children/incapacitated etc.

Yes I want the person/s in power to share my morals. What is good for everyone is their individual rights. Why limit ur thinking of a princepels with in a gentic group. You say reproducion but that does not pass down belifs, insted think about converting people too ur ideaology as that, not reproduction spreads ideals. Which can contain ideals to protect those also in ur group.

reproduction does in fact spread ideals, allow me to explain

first of all, many traits like personality and IQ are partially heritable

Islam spreads through breeding.

The yearly number of converts and apostates are pretty much the same.

secondly, those who get to breed are those at the top of social hierarchies

Yes IQ and Personality are partally heritable, but its not 100%.

third, the people at the top of social hierarchies are those that most embody the ideals of the group

those that embody those ideals have traits that allow for it

they get to breed

and pass on those traits

oh and those traits affect what one's principles are

I did not inhert my parents ideals, and those far from my gentic pool share my ideals.

so, the principles are then passed on

the ideals are then protected by them for the sake of retaining the position of themselves and their progeny in the social hierarchy

of course, these actually need to be effective principles or else the group perishes against nature

or other groups