Hi! and
to YT! I would just keep having her bring the puppy over and keep introducing your little ones to him/her. I also have 2 yorkies, mine are both female, Sadie is 3 1/2 yrs and Lillie is 13 months. My parents watched her from the day I got her (I had to work) and they have a mini schanuzer. So, Sadie had been brought up with another dog. However, she is dog aggressive...this is something we are working on but, the trainer told me that this is just her nature and that I may never be able to change her. She is ok with Lillie.....now....but, when I first brought Lillie home Sadie was VERY upset.....for quite a while...they get along great now but, it took a few months...It's that alpha dog thing...
So, I guess what I'm trying to say is that, if you step daughter keeps bringing over the pup they may get used to her/him and then again they may not. I would just make sure you supervise the visits...you don't want them getting hurt.
Good Luck!