I am thinking that the child does not meet the residency test since the father lives with his wife and son only on weekends and holidays in Mexico. I attended an IRS webinar this Thursday and I did ask this question but I did not get an answer at all. I have read the rules for CTC/ODC and I just see that the child must have lived with tax payer for more than six months. I am assuming that they must lived for over six months in the USA and not in Mexico.