Not dogs. Nobody said every dragon’s counterpart is a dog… yet.  
The teenage dragons are much easier to translate into humans and place in a High School setting than Spike, and it would be easier to include them in the background as humans than as dogs. They are already very anthropomorphic and the right age. The same way the diamond dogs are humans, it would simply be something we aren’t expected to question. (besides, two wrongs don’t make a right)
 
As for the picture, I do plan on drawing Garble’s father as well. For now it is only him and his mother.  
For having been raised by a severe mother with unreasonable expectations, Garble’s mother, Caldera is actually rather lax. This doesn’t say much. If she so much as suspects her son didn’t try his absolute hardest to excel, there will be no leniency.  
Her heart is in the right place, though. She only expects as much as she does because she is fully confident in Garble’s abilities, perhaps a little too confident at times.