The Qualifying Child Relief (QCR) is at $2,000 per child, up to your 4th child. The way a tax relief works is that it is used to offset your assessable income. That is, the $2,000 per child is deducted from your declared income. After deducting all other reliefs that you can claim, the final assessable income is then taxed. (In contrast, a tax rebate is like a voucher you can use for paying taxes. See this post for an illustration.)
A point to note is the QCR can be shared between the husband and the wife. Due to the progressive tax structure of our tax system, I recommend that the full QCR relief be used by the spouse with the higher assessable income (after other reliefs). In short, if you have a higher income, convince your spouse to let you use the QCR. Don’t share.
The other relief is the Working Mother’s Child Relief (WMCR). The WMCR is set at 5%, 15%, 20% or 25% of the mother’s income if she has 1, 2, 3 or 4 children respectively. For example, if a working mum earns $80k and has 4 children to support, her WMCR is $20k.
Note that the total of QCR and WMCR cannot exceed $25k.
For more details, please refer to the IRAS website.