Just because her tail is wagging doesn't mean its in a friend way. Dogs use their tails to communicate. The position of the tail and the way its wagging has different meanings.
I would not allow this behavior at all. She owns NOTHING. You own EVERYTHING. If she's sitting on your son and growls when someone approaches him then he should remove her from his lap, get up, and walk away from her. If she sitting on the couch and she growls at someone who tries to sit down then she gets taken off the couch. Anything she growls over gets taken away. Any person she is "owning" puts her down and walks away. She has to learn that if she growls or tries to "own" anything or anybody then she gets nothing and is left by herself. She only gets treats that you give her, and you can take them away at any time. Everything is yours, and she only gets what you decide she can have.
When Jemma puts her mouth on you in the way you describe it seems to me like she is telling you that she is in charge, the boy is hers, and you need to back down. When you take your hand away and back off, she wins. If a dog did this to me i would "bite" them back, kinda like what Cesar Milan does, by taping them on the neck and making the shht sound. This kinda snaps them out of the behavior they are doing and you get the upper hand. It should make her remove her mouth from your hand, then you pick her up off your son and put her down. Then you sit next to him and "own" him.
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