'them' is wrong because it can't be used as a determiner:

Can't you give me one of them big balloons?

What about here? It's obviously wrong, but not for the same reason. Must we use 'those' when it has a post modifier?

I want to give the big balloons to them who are most helpful.

Can someone please shed some light on this 'them/those' issue?
I don't think "to them who" is "obviously wrong", but to me it feels mostly literary, or a tad old-fashioned-sounding (like something you might read in the Bible). It seems out of keeping here because it doesn't fit the style and content of the rest of the sentence.
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Hi, Mr Wordy

Thanks for your reply.

I just found something relevant here.

The principal reason for our failure was quite apparent to those/them whom we had
brought into the venture.

English requires that we use a demonstrative pronoun (those) to point to or determine the referent for the relative clause.

I think there is a pretty limited range of cases in which "to them who" seems viable. Old-fashioned-sounding literary or poetic sentences like these examples I just randomly Googled seem possible:

"Woe to them who would not open their hands, unconditionally, and cause their brethren to cry out against them..."

"He is only the inexorable Judge to them who harden their hearts in their iniquity..."

And another one I just made up: "Death to them who oppose me". I think attempts to use this pattern in what you might call "ordinary" sentences, as in your examples, are likely to fail.
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