What are the main differences between the children and the master? Observe the physical appearances, dress, and behavior. What can you infer from these differences?

The children of the workhouse really couldn't be more different from the master in terms of physical appearance. The stark outward distinctions between the children and the master illustrate who's in control—and for whose benefit the workhouse is being run.
The master is described as a “fat, healthy man,” while the children are pale and thin from not getting enough to eat. The children are quiet and timid, not like most children at all. This is because they live a miserable life in which they're expected to work all the time instead of playing and enjoying themselves. The master, on the other hand, is an overbearing figure; his very presence intimidates the boys. That's why they draw straws to pick who's going to approach the master and ask for more gruel, as everyone is too scared to put themselves forward.
They have every right to be scared too. In the workhouse, the master's word is law. He gives the orders and woe betide anyone who ignores that simple fact. Once the master has recovered from his initial shock at Oliver Twist's outrageous request for more food, he tries to whack Oliver's head with the large ladle he's holding. If we didn't already know who's in charge, we do now. Discipline in the workhouse is ruthlessly enforced with acts of physical violence against the children. The master's instinctive reaction to what he regards as a challenge to his authority is to lash out. He doesn't even think twice about hitting a child; violence against children is the norm in the workhouse.