I don't buy it. Even the most alert guard dog would not act this way around strangers if it was well-socialised.
This is not entirely true. There are reasons a dog would bark and not really stop when someone they don't know well comes into their territory. You are, for the sake of this discussion, a stranger to the dog, even if you've been on the premises several times to sign paperwork. Dogs bark to alert, to guard, or because they are over excited and want to play. There are other reasons, too, but these are a few of the basics. Until the dog is properly introduced to you and feels that you are no threat, it is NOT unusual for it to bark, even if it has been socialized.
I mean, if you put it into human perspective, even if you (as a person) have been taught manners, you are not going to be friendly with someone you are either afraid of or unsure of, or uncomfortable with.
So what to do? As I see it I have three options, but all suggestions are welcome:
I don't vote for any of these options:
1) Should I kindly ask my land-lady to only let her dog out into the garden at designated times, so that my son and the dog will never have to meet?
She's the landlord, it is also her garden, and you will likely sound very demanding if you, as soon as you move in, try to set boundaries on when she's allowed to let her dog out. IF you phrase it as more of a favor if she wouldn't mind bringing her dog in when you have your son outside that MIGHT sound better.
2) Should I try to encourage her to spend more time socialising her dog to people and other dogs, so that in time the dog will be happy to meet and greet my son?
This will likely be offensive to her, as if you are telling her she has done a poor job with her dog. That doesn't start off the landlady/tenant relationship off well. And, really, you don't know that she hasn't done a good job with this dog, it's just your own perspective.
3) Should I offer her to spend some time with her dog and provide some much needed training and socialisation myself before introducing it to my son?
This is closest to right, in my opinion, except I wouldn't refer to it as YOU training the dog and socializing it, I'd just consider it as "getting to know" the dog. In fact, I really wouldn't train HER dog at all. It's not your job, and even in families or households, dogs that receive inconsistent training by members of the family that have different opinions on training end up confused and the training doesn't work all that well.
Many thanks in advance.