If it was myself, I'd be contacting the breeder. If that wasn't an option I'd get a trainer out to look at the pup not just calling them, it's hard to advise without actually seeing the pup.
Definatly keep them seperate.
Mastiffs are known for beening gentle with children, although they dont always realise they're size. So I'de be very conserned with such a younge pup showing signs of aggression.