My humble opinion is that all dog's regardless of size and breed can be yappy and very aggressive, 90% of that comes down to the training that the dog receives, it is a dog and as was stated above it isint a toy or an accessory no matter how cute or tiny it is.
alot of the little dogs (i.e chihuahua, dashund, miniature poodle) get away with the more aggressive behavour that a larger dog like a Labrador or Alsatian wouldn't simply due to the fact that they are so much smaller. many times iv heard people tell me that their hypo and aggro little dog is fine the way it is because and i quote "when [insert name here] bites it dosent hurt" i just turn around and say if that was a Labrador or a Rottweiler you would be calling for it to be put down.
the basic moral is all dogs can be yappy and all dogs can be aggressive, it is MOSTLY down to the training.
some species are more placid than others but every dog and every animal has a breaking point and all will bite so to speak, im sure if they could our fish would have bitten us all at some point.
i would recommend a spaniel for a situation where there were alot of children, they have soft jaw's and blunt teeth as they were bred to retrieve waterfowl so if they did turn around and bite the damage would be alot less than a chihuahua or miniature poodle which have far sharper teeth. they are also alot less active than either of those species.
Just my 2cents, please feel free to pick apart what i have said as it was only based on personal experiences.