First and foremost, this case is very different from David and Seans. The only similarities are the countries involved and specific laws that apply (Hague Abduction Convention.)
1.) Both parents appear to be Brazilian (not positive about father.)
2.) Both parents have lived in both countries and appear to be comfortable and familiar in both countries.
3.) Both parents are still alive.
4.) Children may speak both languages.
5.) Both sides of the children's extended family may live in Brazil.
On the one hand American courts should take into consideration the effectiveness (or lack thereof) of Brazilian courts at securing access for American parents after children are returned. On the other hand Brazilian courts have already shown in this concrete case that they are, or at least were, willing to allow travel to the United States for visitation.
This is the type of case where I'm actually somewhat less against calling this an "international custody case" rather than an abduction. It would appear neither parent is being forced or asked to go to a foreign country they know little or nothing about, and have no friends or family in, to try to litigate in a completely alien court and legal system where they have to face nationalistic and xenophobic biases.
The Hague Convention case is going to essentially revolve around determining what the children's habitual residence was at the time of the "unlawful retention" when the children were not returned to Brazil.
Determining habitual residence can be complex in cases like this. It requires a fact based analysis of which country the children have ties to and what the intent and expectation of the parents were. Courts have held, rather reasonably in my opinion, that when both parents plan a permanent relocation of their family to a foreign country that this "shared intent" and action creates an immediate change in the "habitual residence" to the new country. Sometimes, however, parents agree to a "trial period" or something similar so the courts will likely look for evidence as to what the parents expressed intent was prior to moving back to Brazil.
It will be an interesting case to watch, but with both parents seemingly having strong ties to both countries this case is much less clear-cut than many of them.
What does appear clear here is that the father did take the children on vacation to the United States and refuse to return them. I think that, generally speaking, this is a "legal strategy" that should be strongly rejected.