If that's anywhere like where I live, "lower class" means people that don't pay as much taxes but on the other hand end up having even less money left. Yet, lower class people tend to have more children and fertility has a negative correlation to income. Perhaps it's a matter of education and ambition rather than the economic challenge.
I wonder if some if it is also the capital intensity of children, the higher the monetary investment you bring (clothes, gear, schools, activities) the harder it is to have lots unless you have the money.
The government is practically killing the population in a Darwinian sense, discouraging intelligent and educated people to have kids and encouraging the others to. It is the way to the movie Idiocracy.
Yet we see the same tendency here. Lower income families tend to have more children despite earning less in net. So I think you have to at least consider the possibility that it has less to do with the financial hardships, at least in absolute terms of income, than with some correlated factors in your home country.
For example, the low income population here are generally less educated. Perhaps people end up more aware of the consequences of having children through education. Maybe there's an underlying factor that leaves people with both a propensity for academic achievement and an inclination towards having less children, like time consuming dedication to an interest or a career. Perhaps higher income parents have higher and more costly standards for their children.
I don't know what country you live in that the middle class generally ends up with less disposable money than the lower class through wealth redistribution programs, but it's not the case here or in many other countries where the negative income-children correlation can be observed.
I won't comment on darwinism or the idea of Idiocracy as a realistic outcome of any kind of selection process other than to say that it's stupid. If you are able to have children but won't, you have yourself to blame for nature selecting against you, if that's at all an issue to you.
Regarding to "If you are able to have children but won't", that applies to uneducated people; if you can have kids and you won't because you have not enough money to raise them properly, pay for education and health care (state health care it is free and almost worthless) then people decide not to have kids in such conditions. If you are uneducated, with drinking problems and you don't care about kids, you have as many as you can so you can drink all the money that you get. Not kidding and not exaggerating at all.
Reading this part made me think "which other country is like Romania?"
Sure enough, the author is from Bucharest, Romania.
We pay 25% for a pension that may never come. The state also computes pensions in magical "points" which means they can arbitrarily tweak in the far future how much of your current money you see back.
We pay "only" 10% in income taxes and some/many IT positions are exempt (so far).
Once we have our salary in hand, we pay 19% in VAT.
I'm guessing you haven't lived in Belgium, Germany or Scandinavia.
One year ago I got an offer to work in France, so I made a comparison: tax rate is a lot lower there for a similar income. At the same time inflation rate is higher in Romania, credit is a lot more expensive, corruption a lot higher, so the taxes are too high for what you get.
[1] https://wol.iza.org/uploads/articles/228/pdfs/female-educati... [pdf]
My dad's family had nine and he grew up in the city in a small house. I think religion may have some part in it for example Roman Catholic where contraception is frowned upon. It was quite common to have a dozen children. My mother's family who were farmers for awhile (then a restaurant) also has nine children.
Government financial support may also play a role. My country Canada has many social support systems that allow larger family sizes. I'm not criticizing it but the support is there for the children. It would be curious to see family size versus government support.
It's completely prohibited.