when my two boys (G shepherd & Staffordshire) started nipping as puppies, I bit them back harder (tho not that hard to break skin!) & growled loudly at them, showing my teeth. They understood that I was a bigger dog than them and knew best (and I could bite....they understood that too). Equally, they got tonnes & tonnes of love & pats when they were good. They stopped biting after only one or two lessons & became such beautiful well mannered dogs that everyone loves.
Remember, you are the dominant species. Dominate them, hold their gaze and their head if you have to & talk to them in "dog", eventually, it will pay off & they will become contributing family members who are happy because they know your rules ...& dogs love nothing more than to please you.