Every child is a great blessing from Hashem. There are people who worry about being able to afford many children. But Hashem is the One Who blesses us with the children and He takes care of the children's sustenance. The Lubavitcher Rebbe said that every child is born with their parnassa already taken care of. If Hashem gives an additional child, He certainly provides for that child. Some people think if they did not have an additional child if they would have more money: on the contrary, each child brings an additional blessing. It is a lack of faith to think that Hashem cannot provide enough for every child.
Below is a beautiful video of the Rebbe speaking about parnassa and about family planning:
If a couple has not had the merit to be blessed with children, besides the physical or medical issues that need to be addressed, spiritually there are certain things that should be done to make sure the pipeline of blessings is open.
Certainly the couple must learn and observe all the details of the laws of taharas hamishpocha, family purity (mikvah).
And of course they should check their tefillin and mezuzot in the home (If they do not yet have mezuzot or tefillin, that should be arranged).
And of course it is important to give charity, to say extra tehillim and prayers, and to pray by the gravesite of tzadikim (as well as getting blessings from Rebbeim or grand rabbis, and even simple laymen who also can bless: as the Baal Shem Tov taught, one should never look down at the blessing of a simple Jew because every Jew can bless and one never knows which prayer or blessing will be answered so always answer amen).
May Hashem bless everyone with children: good, healthy and pure children that will bring joy to their parents and to Hashem.