Is it possible to use tax relief for children on a second employment contract, not the primary one, if the taxable base from the first contract is not sufficient?

No, tax relief can only be applied through the employer who conducts the annual personal income tax reconciliation, and this must always be the primary employer. If the taxable base is insufficient and you have other income that was not included by the first employer, then you must submit a tax declaration by April 30, 2025.