It is halachicly prohibited to refrain from having children due to financial considerations.
Um… Source?
DaasYochid – It may be a mitzva to have children, but it is not technically more than a mitzvas asei. One does not have to turn himself into a pauper for a mitzvas asei. Why should this mitzvas asei be different? And even if you give a satisfactory reason, that should at least not obligate one who already has a son and a daughter to have more children if he cannot afford to.