This seems like a completely useless handout to the rich. I can agree that the income limit should be raised, as 100,000 baht per year household income is just 8,300 baht per month which is ludicrously low. But well-off Thais have no need for an extra 600 baht per month and it certainly won't encourage them to have additional children.
This government seems hell-bent on handouts to the rich, first with saying they want to decrease income taxes but increase VAT, which means a tax increase on the poor who don't pay income taxes but certainly pay VAT, and now this handout to anyone with a child rather than either increasing the subsidy to the poor, raising the income ceiling, or both, making better use within the same budget.