How can I teach my 9 week old Yorkie puppy to stop biting & chewing on my toddler?

I rescued a Yorshire Terrier puppy 2 weeks ago. I haven't owned a dog in about 8 years so I'm a little rusty on puppy training. He is still very small, only 3 lbs. He's doing very well in all areas so far EXCEPT for biting & chewing on my 1 year old & 3 year old children. I know he is only playing, but he will not stop no matter what I do. I've tried saying "No" everytime he he does it and giving him a chew toy instead. I've tried putting him in his crate for a few minutes after biting. I've also tried yelling "ouch" & leaving the room for a moment. He continues chasing my children and biting their feet, legs, etc... Any suggestion?