Toni Collette, who plays Cole's mother, is from Australia. And while she does a very good American accent, if you listen carefully while she is pushing Cole in the shopping cart,(DVD time index 59.50) the yell she lets out sounds very Australian.
If I am not mistaken, the only thing that Cole's mother yells when she is pushing him is "AAaah!!" Where in the world are you hearing an Australian accent in that? I would think that an Australian "AAaah" would sound the same as as a American one. I would understand if she had said "AAaah,mate!!" or something. Really, come on now. I have to aggree with TRP on this one. This is nitpicking if I ever saw it!
You are talking about dialects, I'm talking about accents. Accents shape the muscles of the throat and mouth that causes even the non-linguistic utterances to have a certain inflection. A good example of this is the Nazi's in raiders of the Lost Ark -when they went ARRGH!(usually when being killed) Spielberg made sure they sounded German. And while this Sixth Sense slip-up is a very small one, it is still interesting to hear -if you know what to listen for.
For all we know she may have lived in Australia when she was younger and moved to the States. Not necessarily a mistake as the characters past has not been discussed.