Re: 2 of my kids didn't get the imaginary friend doll.

For the 'try for baby' children the doll will be send in the mail. I'm not sure if every child gets one.

It's not possible to buy one or cheat one in. I did find a mod through Reddit:
https://www.reddit.com/r/thesims/comments/3973w7/is_there_anyway_of_getting_an_imaginary_friend/

I don't know if this mod is safe, so I would suggest doing some research. And backing up your game before using it.

On Crinricts blog you can read how to do that:
http://sims3.crinrict.com/en/2011/03/faq-what-files-should-i-backup.html