Hello and welcome to Just Answer. I have 20 plus years experience in all areas of US taxation including multi-state and international issues.
Yes, the federal tax withholding is increased due to the higher Federal taxable gross income, and the higher taxable gross income will be reported in Box 1 Taxable wages on your Form W-2. My understanding is that registered domestic partners in community property states such as California must each report half the combined community income earned by the partners. Therefore, you and your partner will each report 50% of your combined total income, in addition to any separate income you may have.
Since I don't know your personal tax situation, I can't tell you if you are receiving a benefit from this. If each of you earn about the same amount of wages, there probably won't be a significant difference in what you would normally pay in taxes. However, if one of you earns substantially more than the other, then the one earning less will be paying more in taxes, and the one earning more will be paying less. This may still provide a benefit when you look at the combined taxes for both partners and what the combined tax would have been without dividing the community income.
Since you do have a dependent child, the best tax benefit would probably come from being able to claim head of household status, but this means that one of the partners must be able to show that they provided more than half of the support of the child. For more information you can see IRS Publication 555, Community Property and the FAQ at http://www.irs.gov/newsroom/article/0,,id=245869,00.html.
I hope this helps you with your question. Please let me know if you need further assistance.