For some reason everyone seems convinced that Alex, Danielle's child, is a son/daughter.
I'm not sure if this has been brought up, if so I'm sorry to rehash old news but there was some distinct wording that got me to thinking about this possible "theory".
Danielle refers to Alex being her "child". I thought that was strange. In all of my life I've never heard my parents introduce me as their child. It's always "my son". I started talking to some of my friends that were parents and they came up with the same realization. Maybe I'm splitting hairs here, but isn't that what we're here for anyway.
Let's say Alex is the thing we've been referring to as the "monster". If Danielle was responsible for it's existence then she may feel as though it's her child. She may also not know it's sex (if it has one)...this could be why it's not referred to as her son or daughter. It would also be given the name Alex, which has been given to both males and females. Also, if Danielle felt that the "monster" was her child...she may take offense to it being referred to as a monster. This giving us her quote of "there's no such thing as monsters."
Just something we've been throwing around here in the office.
I have been pondering this idea for let's say thirty seconds and you are right, I have never heard any person refer to their kid as their child. This happens quite a few times in books but not usually movies or shows. I say movies or shows because these too are written just like books are. I do not mean format. *So sometimes while writers are writting, he or she may use the word child instead of son, daughter, or kid. When scripts are re written into the shooting script, the original words of the writer are usually changed into a real conversation rather than written words. I hope you understand what I mean people. The reason I made this point is because this may help validate Onetre's theory of Danielle being responsible for the creation of "The Monster".
Here's what I think - they used a unisex name (Alex) and had Rouseau say 'child' so as not to give away if the person was male or female. She might have simply said 'child' because English isn't her first language and that's just the word she said. She hasn't conversed with anyone in English (or at all) in a long time, remember?
If Alex is the monster and also in some way Rousseau's child, it would only make sense that she would be wondering where it was. As far as we know, the monster has not been around for a few days (a few episodes anyways), since the encounter with Locke. Could she have been asking Sayid about it because she also has not seen it recently? Also, if the monster is gone, where did it go?