It’s better if the other files/provides the parent to their employer because they have a higher annual taxable income.
Yes, you can benefit from tax relief when you are with your employer and file an annual declaration after that.
If the employee leaves before or during December, they will claim their tax relief through their next employer by providing a certificate under Article 45 of the Personal Income Tax Act from their previous employer. If they will not have a new employer, they will need to submit an annual [...]
The employee has the right to claim tax relief for children through their employer, even if they will be filing an annual tax declaration. In the annual tax declaration, they will simply need to indicate the number of their children in the appropriate section.
You need to add up the tax bases from all your payslips for the entire year. Each payslip has a line showing the tax base for that month.