1. It is quite reasonable because in these kinds of places kids sometimes are considered as an additional source of income.
Is "are considered" the passive?
2. Are both words in brackets possible or just one and if so for what grammar reason?
One part of people (believe/believes?)
Another part of individuals (feel/feels?)
One part of people believe that an advance can be measured by the amount of money possessed by an individual.
Another part of individuals feels that if one brought up a few kids than he or she may be declared as a successful person.
Is it connected to the subjunctive: "englishclub.com/grammar/verbs-subjunctive.htm"?
Your second question cannot be answered, since the phrases you cite are not English.
Try amending to 'Some of the people believe...', or 'Some individuals believe'.
Other more formal options include 'One section of the populace believes...'.