I doubt you would get through the First Wing into GF at Terminal 5. You would probably have more luck at Gatwick. In Terminal 3, the Qantas lounge sounds like the place to go - both because you would actually fall within the policy (they don't seem to count kids at all), and because it's probably the better suited lounge for small children anyway.
At out-stations - who knows; depends what mood the lounge agent is in, and how busy the lounge is.