On the part where the little boy notices that there's no name on the orange juice carton and he goes upstairs and finds his mom and dad having fun and laughing together, at first, his mom has long hair. Then when the camera goes back to her, she has short hair!
I've seen the movie... and I must say, it's adorable... but on your slip-up, I think it wasn't a mistake. When he walked in the room... he said he saw two strangers laughing together. If you will notice, neither of the characters are the ones who play his parents in the movie. It's kind of showing the younger version of both of them, laughing and having a good time together, like they used to do. Then when it flashes back to them it shows the original characters who play the parts. I don't think it's a slip-up... I think the woman with longer hair is the younger version of his mother that he sees.