How can I retrieve Current Relational Data from DataSet

I have two tables
tblEntity [ Entity ID , Entity Name ,........]
tblUser [ UserID , Entity ID , UserName , ............]

I have created DataRelation "entityVuser" in between these two tables and successfully added in DataSet. I have two comboboxes one is showing Entity Names and when i select an entity the second combobox shows its related Users through “entityVuser" relation.

Can I retrieve such users from dataSet directly? Let’s suppose I have a function
comboBox_EntityNames_SelectionChangeCommitted( ....... )
Q1.when i select an entity from EntityNames combobox this function will be fired and i want to get list of all users related to selected entity.
Is it possible?
I m curious about the architecture of ComboBox ( DisplayMember and ValueMember)
for example

Code:

comboBox_UserLastName.DataSource = m_ds; comboBox_UserLastName.DisplayMember = "Entity.entityVuser.UserLastName"; comboBox_UserLastName.ValueMember = "Entity.entityVuser.UserId";

How Combobox retrieve such relational values from data set.

Is there any way like, I get child table data directly from dataset?
m_ds.Tables["Entity.entityVuser"].Columns["UserLastName"]