"buy... clever" & "buy... miserable"

You can’t buy your kids clever. What’s more, if they’re merely above average, by sending them to some hideous Holland Park hothouse, you’re probably buying them miserable. — Alex Proud, "Your child is not a genius. Get over it"

As far as I know, clever and miserable are both adjectives, not nouns. So why were they used with to buy? Is this an idiomatic way of saying You can't buy your kids intelligence and you're probably buying them (moral) misery? If so, what is the idiom used here? I'm not familiar with it.

And (adding to oerkelens' statements about meaning), analysing the grammar involved, you're quite right, clever and miserable are still adjectives, referencing the (resulting state of) the kids here. This makes the usage a (resultative) transitive link-verb structure. However, it is a non-standard example, akin to

Quote me happy.

More common examples are:

He hammered the metal flat.

She shot the gangster dead.

The ants were eating the man alive.

(the last depictive rather than resultative)

Buy them clever and buy them miserable means that by spending money, you make them clever (or miserable).

In this case, what you buy them is education.

You can't buy your kids clever.

Simply means

No matter how much money you spend on education, books or training, if your kids are not clever, no amount of buying stuff will make them clever.

For the grammatical construct that is used, see Edwin Ashworth's answer :)

This is a case of the author using adjectives as nouns as a sort of grammatical liberty, as you suggested.

You can't buy your kids clever

implies that "clever" is some sort of object, presumably the state of your kids being clever. And

you're probably buying them miserable

means that "miserable" is the same sort of object, again, the state of your kids being miserable.

If you wanted a more strictly grammatical version, you could say:

You can't buy your kids cleverness, but by sending them to some hideous Holland Park hothouse, you’re probably buying them misery.

which is pretty much what you thought in your question.