I think you are right that you may as well get the spousal visa before you come over for the first time...it will save potential hassle at immigration that you may get if you came as a visitor...
"I'm just coming to visit"
"yes, my husband is british and living here"
"yes this is his baby"
"yes, I will be looking for a job while i'm here"

Check out with the consulate how long it will take for Emmie to get a passport. If she can't get one in time, I suspect you'll need to get her a visa.
You are right about your income counting when the time comes for the other kids to come in.
Vicky