Recently me and my wife became naturalized US citizens (after being on green card). We were told by USCIS officers during the interview and Oath ceremony that our child (minor below 18 years age and holding green card) is now automatically an US citizen. Do we need to apply for N600 (Certificate of Citizenship) for child as a proof of citizenship or would a US Passport be sufficient for current and future purposes? The fees for N600 are just astronomical and would not make sense if this document is optional. Please advise. Thanks.