Is there an effect both positive or negative to the county's economy when couples have plenty children?
As you said at the end of the post, immediate effect is spending - more spending goes toward the trial, more work output required to meet that expense.
The positive effect on the economy comes later on in life when they are able to start working, unless there are not enough employment opportunities or how the children were raised causes complications with employment.
In the past before the current economic era, around the 15th century and the beginning of the French industrial revolution, having many children was seen as fertility and good for both the family work force and country's also. The labour force was majorly manual and those who worked at the plantations were sort after as able bodied men or women.
Much different time era...the fact that fertility was considered good not for love, legacy or family reasons, but for an economy purpose, says enough about that time. People were slaves...we still are now in a more modern way, but not in a way where we are looking to populate for more workforce, lol.
But at the moment, what is the viability of having many children especially as it could reduce the per capital income.
For the average person in the global economy today, I'd say a child would be a challenge to say the least. If you are wealthy, maybe it's a different story. Though this is a much smaller class of people in comparison to average, or poor.