Population will continue to stabilize and possibly fall in the developed world and population growth will lessen in the developing world.

Also, a life on basic income would probably be similar psychologically to unemployment -- more depression, disease, shorter lifespans, etc. These things contribute to a reduced rate of childbirth.

Not sure - unemployment would cause a lot more anxiety, I would imagine. Are pensioners especially depressed (if age and diseases are factored out)?

It has to be only for adults. Or else people are going to have many children to get more money.

And it has to be only for native citizens, to avoid mass immigration.

It would be difficult to let kids live in squalor, though. Presumably parents would receive at least some extra money, but perhaps it would decline with the number of children.

More people requesting entrance isn't going to magically increase the quotas all developed nations have for residence.

I don't see countries with a growing population being able to implement that at all, not because of the rising population, but because they are poor.