You all worry about mammary cancer in your unspayed dogs, but what you should be concerned about is pyometra. The more times your dog goes into heat the greater the chance of contracting it. We lost our dog at age 5 because of our ignorance. She got it at age 4 and had to have a hysterectomy. A year later she started failing. I don't think she ever truly recovered. Here's a link so you can read all about it. This is why I took my 2.7 pound Debbie to be spayed as soon as she was old enough.
http://www.marvistavet.com/html/body_pyometra.html
As far as your son with the yorkies, that sounds adorable. To be safe, I wouldn't think it's the best move to mix little kids with tiny dogs, but it sounds like he's a gentle child and you supervise things very carefully. It's a shame not everyone is like you.