I can't stress this enough. It is IMPORTANT that you read the instructions for filling out the W-4, or you will risk under withholding of taxes from each paycheck, resulting in YOU paying an underpayment penalty at tax time. For example, if you have dependents, and none of them are under the age of 17 on Dec 31 of the tax year, then you will enter a ZERO in box E of the W-4. So just read the detailed instructions and you can't go wrong. If necessary, you can read those instructions online at https://www.irs.gov/pub/irs-pdf/fw4.pdf
Your girlfriend's 7 year old does not qualify you for a Child Tax Credit or Earned Income Credit or for Head of Household, because he is not related to you and cannot be claimed as a child dependent by you at all but as an "Other" type of dependent, if you meet those qualifications. Either you or your girlfriend can claim the younger child but only one of you can claim anything related to that child, such as Head of Household status, dependent, and any child related credits. You and the mother may not split anything related to that child between you.