"Él no trata bien al niño."
No, the sentence could be:
Él no trata bien al niño = He does not treat the child well.
Él no lo trata bien = He does not treat him well.
"Lo" would replace "al niño", so it would be like saying it two times, as in "He does not treat him the child well" or something similar.
This topic is something I have a lot of difficulty with. For the sentence "He loves them" you can write "Él los quiere" and if you want to clarify further you can write "Él los quiere a ellos. It has been explained that you can leave out the "a ellos" but not the "los". This confuses many English speakers (myself included) because most times if you include the clarifier part, the object pronoun in this case "los" seems redundant. It has been explained that "that's how they do it in spanish" which I get, but I am always unsure when to add the object pronoun. Some sentences don't seem to need them. It is unclear to me when/why you should (or shouldn't) add the object pronoun.
Yes, I have noticed many people seem to have trouble with this. Being honest, "él los quiere a ellos" sounds weird to me, I cannot assure you it is incorrect, but I have never said that.
Thing is, you already know the rules and the only thing possible is to remember and apply them.
The general rule is that you either write down the direct object or the pronoun replacing it, but not both since, as you say, it sounds redundant. "Quiero el libro / Lo quiero", "Miro la ventana / La miro", NOT "lo quiero el libro" or "la miro la ventana".
Now, it is different when we talk about indirect object pronouns. You may drop the I.O. and just write the pronoun if you know who you are referring to:
"Les he dicho a mis hermanos que llamen" = I have told my brothers to call. If you have been talking about your brothers for a while, you would just say "les he dicho que llamen" and that is it. You cannot say "he dicho a mis hermanos que llamen", though, as you have already learned.
Why do we duplicate indirect objects? No idea. Maybe it makes no sense (grammatically it probably does not), but it is just the way it is.