YANBU-not about the turds or about the fact that they have a dog where you are not supposed to have one.
YAB V R by turning a blind eye to the dog until this happened, it is risky to have a dog where they're not allowed anyway, but if you are going to have one for sakes you clean up the evidence of it!-Disregarding that it is against the law not to.
Regarding the breed, it could be that the dog is totally harmless, and some breeds that look cute as hell can be as harmful as those who look menacing, and vice versa but I wouldn't want to take the risk I would want some reassurance.
If I was you I think I would write them a note express your concerns, stress that you are concerned about the breed, and concerned about the fact that they appear to not be cleaning up after the dog. Make it clear that until the dog mess became an issue you were happy to turn a blind eye, as it were. Keep a copy for yourself if you write a letter-in case you need to escalate the issue to the landlord.
Depending on the response or lack of, I would then go and see them and judge for yourself.
It's healthy for kids to be wary of dogs although being terrified of them isn't-Most dogs will not attack or hurt anybody unless they're given reason to-but nowadays it seems a lot of owners give them reason to or even train them to :(
I would judge a dog on the owner-if these people do not respond to your letter, or respond adversely, I would knock on their door and see how the dog behaves and how the owners behave-is the dog excitable and friendly or is it aggressive, do they discipline it, or frighten it, if It's the latter I would have concerns.
Even if It's the friendliest dog on the planet, though-dog mess can be toxic and they should definitely clean up after it! I have a large dog and I know unless seriously provoked she wouldn't hurt a child or anybody else, but I sure as hell poop scoop!