I've been taking my DD to WDW on my own since she was 6. The first year, after much jumping through hoops of fire, I managed to get a standard letter of permission from my ex husband to take her out of the country. I then went back to peruse over my divorce decree which has no restrictions or clauses prohibiting the same. It simply states that I have sole residence and custody of the minor child. I work for a legal firm and simply had one of the partners notarise my decree of divorce and have taken that with me ever since. DD does have the same surname as me as I never bothered to change my name when I got divorced, which helps. I was only once ever asked if I had consent and I said yes. Started to reach for the letter and the customs official gestured dismissingly to me to leave it.
I think once the children are old enough to communicate efficiently and answer a couple of questions if need by, they worry less. It is not a requirement, according to the CBP, but merely recommended.
For peace of mind you probably have a Court Order awarding you residence or with a clause about international travel. I would just take a notarised copy, or even the original, in the unlikely event that it is called for. If you are on speaking terms with your son's dad, there is a basic style for a letter which he could sign if that makes things easier.
The US border officials are not interested in, nor do they have time to become involved in upholding holiday arrangements for separated parents. They are more concerned with what could be an abduction in a separation dispute and therefore, if one or both of you has a US passport or residence permit for the US, then you would have more to worry about and would likely encounter closer scrutiny. I am assuming by the very nature of your question, that you have neither, therefore this should not be a concern of yours. I do like the peace of mind of having my legal documents to fall back on, but hardly gave it a thought by last year and nearly forgot to take them.
