I have a 6 month old neutered male and I just brought home a 4 month old little girl. My male is a big kid and loves to play non-stop. he was a little rough with her at first and he is bigger than she is so I had to rescue her a few times when she yelped. When this happended I would make sure they had a tug of war toy or one where both could have part of it in their mouth. The other thing I would do is play fetch and throw one toy a long way in one direction first for the male and then another toy a short distance in the other direction for the little girl to run after. After a little over a week they are best friends. She seems to be able to tell him when he gets too rough and they do pretty well.
My westie and my poodle did better acclimating to the little girl than they did to the male. I think they were used to "new" puppies after he came and they accepted her more quickly, but he is more annoying than she is!!
