And though I know it won't happen, I would love to hear the judge lambaste the uncle for manipulatively luring the woman and her son here, falsely assuring them he was only going to take the boy to go visit his grandmother, and then snatching him.
As you said, if they wanted to make a try for custody, they should do it properly through the courts. File the paperwork. But don't kidnap the kid and prevent him from seeing his mother who has been his sole parent/guardian for the last 4-5 years.

"Marriage is a wonderful institution...but who wants to live in an institution?" -- Groucho Marx