You can't. And, don't file your return until you get one. If you leave the child off the tax return and then plan to file an amended return after they get their SSN, you won't get anything for it. A child must have an SSN on a timely filed tax return, you can't go back and amend later.
If this is an adoption situation you may be able to get a temporary tax ID number. https://www.irs.gov/individuals/adoption-taxpayer-identification-number
Otherwise, contact your child's social worker. They are generally required to help you obtain the SSN from the biological parents, or help you apply for one if the parents never applied.
