UBI still permits people to have children that they'd otherwise be unable to afford, but at least the absence of other sibsidies will force individuals to decide how they'd like to budget that income, on children or otherwise.
> Simply stating something does not make it a fact.
Your first quote makes it sound as if it was the inevitable outcome.
SOME people. Not all. It's a slippery slope fallacy.
What if the same type of people that try to manipulate the system do it because it seems like the only reasonable way to stay afloat?
It could be possible then that some of them would not have more kids to get more money.
Indeed.
A foster care system is, in my opinion and at least in poor areas of large metros - is a good (and pretty depressing) example of what many people are willing to do for money. To some extend - childcare subsidies demonstrate the same.
That said, there's no reason offhand to provide basic income to minors. Parents can choose to have as many children as they want, at their own expense. One of the key advantages of basic income is getting rid of the complexities and irregularities of the current welfare model. Subsidies for children are one of those complexities.