Phasal verb: pick up - separated

[Daltomaciel]

I'm studying about phrasal verbs and I had a doubt:
See those cases:

I picked the children up at school before 6.
I picked up the children at school before 6.
I picked them up at school before 6.
I picked up them at school before 6.

The three ones are okay, but the last one isn't.
My question is: if I can use 'pick up' separeted (sentence 2), why can't I do this in the 4th sentence?

[Barb_D]

With the phrasal verbs, when you use a pronoun, it can't go after the entire verb.
I looked up the word.
I looked the word up.
I looked it up.
* I looked up it - No.

It's fine with regular nouns.

[Lost_Soul]

[Yankee]

[Lost_Soul]

[Barb_D]

I'm not aware of any way to tell "from looking" which type they are -- I'm afraid you do have to memorize them. I wish I had a better answer.