That only works because of the lack of child labor laws and the lack of an advanced economy in which education is required in order to get a well-paying job in countries where extreme poverty is commonplace.
You didn't read parent right: the child will be an adult when the parents are older. The more kids you have, the more likely at least one will be successful enough as an adult to take care of you.
No, I read the parent just fine. The child may be an adult when the parents are older, but that means nothing when the adults are bankrupted trying to give the child an education so that he/she can support the parents, and the child can't work until he's an adult anyway.
Of course the probability will be higher with more children, but the cost of having more children in a developed country far outweighs the benefits.
There's a huge difference in the results when you do it in a developing country and when you do it in Spain.
Just to make it clear: Spain, and the rest of Southern Europe are developed countries with high standards of living; extreme poverty is not even a prospect now.
In societies of extreme poverty children have long been viewed a hedge against old age