Hi and Welcome to YT!
My Cerise will be 10 months on the 26th and she weighs less than 4lbs. She takes Sentinel (brown) once a month for fleas and heartworms and we live in TX where fleas and mosquitos are a serious problem. So far she hasn't had any problems. Your baby is old enough to take Sentinel (brown)
Also what brand of Oatmeal soap are you using? Some may have oatmeal in them but still have chemicals that are harsh on a Yorkies skin. I use a liquid soap that is castile/olive oil base and I bathe Cerise once a week. It's very humid here too and she needs that weekly bath. The liquid soap is non-drying and great on sensitive skin and the Yorkie hair. It's called
Dr. Bonner's Baby Mild Liquid Soap 
I recommend it to everyone that has a scratching Yorkie because it is fabulous! If you don't have a health food grocery store that sells it in your area you can order it online. If you are sceptical get the smallest bottle and try it
