美国入境管理部门（CBP）官方答复是这样的：
父母一方携带18岁以下未成年人入境， 强烈建议 携带另一方的同意书， 同意书内容如下：
I acknowledge that my wife/husband/etc. is traveling out of the country with my son/daughter/group. He/She/They has/have my permission to do so.
同时建议对此同意书进行公证


CBP不一定会要求你出示，但一旦CBP官员要你出示同意书而你没有的话，恭喜，你要进小黑屋了，然后CBP官员会研究这个是不是你孩子，等研究完了之后你才能出来。。。


所以为了避免麻烦，建议开下同意书，公证倒是无所谓。


原文：


If a child (under the age of 18) is traveling with only one parent or someone who is not a parent or legal guardian, what paperwork should the adult have to indicate permission or legal authority to have that child in their care?Due to the increasing incidents of child abductions in disputed custody cases and as possible victims of child pornography, Customs and Border Protection (CBP) strongly recommends that unless the child is accompanied by both parents, the adult have a note from the child's other parent (or, in the case of a child traveling with grandparents, uncles or aunts, sisters or brothers, friends, or in groups*, a note signed by both parents) stating "I acknowledge that my wife/husband/etc. is traveling out of the country with my son/daughter/group. He/She/They has/have my permission to do so." See our Q&A parental consent.* School groups, teen tours, vacation groups.CBP also suggests that this note be notarized.While CBP may not ask to see this documentation, if we do ask, and you do not have it, you may be detained until the circumstances of the child traveling without both parents can be fully assessed. If there is no second parent with legal claims to the child (deceased, sole custody, etc.) any other relevant paperwork, such as a court decision, birth certificate naming only one parent, death certificate, etc., would be useful.

Adults traveling with children should also be aware that, while the U.S. does not require this documentation, many other countries do; failure to produce notarized permission letters and/or birth certificates could result in travelers being refused entry (Canada has very strict requirements in this regard).If you wish to receive automatic updates to this Q&A, select "Subscribe to Updates" on the left side of this screen.

在美国入关的时候，是以家庭为单位的，一次最多不超过3人。

亲妈不用亲爹证明你是亲妈。
同组入关。

美国入境建议看看http://ask.qyer.com/question/736431.html

美国不用的。