First off, I would let your landlord know that her dogs are barking day and night. It would be helpful if you kept a log of when during the night the dogs barked (12am, 2am, 3:45am etc) and whether this is a few barks at a time or if they bark for ten minutes straight.
2nd, ask your landlord what the noise ordinance hours are for your building. 6:30pm is WAY too early to expect a 2 yr old to be quiet. Let your landlord know you are asking about the quiet times because you want to make sure YOU are "complying". It is the landlord's job to make sure the other lady is aware of those same hours to keep her dogs in line.
3rd, yes, do call the police any and every night her dogs wake you up more than once. (You can call the next morning). When you call the police, know what time(s) the dogs barked and how long approximately they barked for. The police will build a file of your (and possibly other people's) complaints against her. I don't know if they are required to confront her on the first time, but if it is a repeat problem, they do eventually address the issue.
4th, let the police know you have contacted the landlord. I would also let the landlord know if I contact the police.
I honestly don't know how she thinks she can get away with complaining about your son when it's her dogs that are actually a real problem. I think she must be one of those crabby people who hopes to annoy you into moving.
Keep any letters she writes to you and document everything! because if she goes completely nuts and tries to do something outrageous like sue you, you would need the documentation. Even write down any conversations/arguments you have with her.
I'm a mom of a 2 yr old. I also own 2 dogs, so I've been on that side too. I have been a renter and I have been a landlord. I have also have issues with a neighbor over noise problems. And my husband works nights.
(Does this lady work nights, and therefore she isn't there to keep her dogs quiet at night, and probably is trying to sleep in the day? Not that her working nights would make her right, it would just shed a little light on the subject.)
I hope some of that was helpful.
Oh yeah, and you can call the humane society/animal shelter also. Sometimes they will come and investigate if they suspect the dogs are not being adequately cared for. Constant or frequent barking might be enough to get the shelter to look into it. (I used to volunteer at my local shelter too.)