How can living in poverty cause children to withdraw fom their peers?

There can be a variety of reasons for
this.

One reason would apply if poor children have peers
who are not themselves poor.  When interacting with such peers, the poor children would
come to realize that they are different from their peers in terms of material
possessions.  This may well cause them to become self-conscious, particularly in a
materialistic society such as that of the US.  They would then withdraw from the company
of those peers.

Another reason has to do with what is known
as emotion knowledge.  This is, in part, the ability to "read" other people's emotions. 
This ability seems to be less developed in many poor children.  This lack is believed to
be connected to their poverty.  The proposed connection is that poor children have
parents who are more stressed.  This stress can lead to the display of a great deal of
negative emotion towards family members.  Exposure to such things seems to make it more
difficult for children to comprehend others' emotions.  Children with this problem are
more likely to withdraw from interactions with their peers.