"I speak to the second child."
Translation:Eu îi vorbesc celui de-al doilea copil.
Forgive me if I don't remember correctly, but: Why is celui de necessary, again? Was it because the dative article like in copilului cannot be attached so far behind in the sentence, and cardinal numbers can't take articles?
Are there alternative forms like primului / "secundului" copil that one could use instead of celui de-al doilea copil?
It's just the way the ordinal numeral works in the dative (or genitive) case: "celui de-al doilea". I suspect the "de-" was added to make pronouncing it easier. You can't ommit it.
"Eu vorbesc cu al doilea copil" is grammatically correct, but it has a slightly different meaning. It's more like "I'm having a conversation with the second child" (ie: two-sided conversation)