Taking the passport really is the best short term solution. Beyond that, when myself or my ex traveled out of the country (to Peru) with our kids, whoever had the kids needed to have a notarized letter from the other parent saying it was ok that we were traveling with the kids. I never took the kids out of the US myself, so I don't know who/when/if it was checked on the way out. But on the way back in, both Peruvian and US immagration officials asked for it. Now I've been told by other people that this hasn't been true for them, not sure what the difference was.
If you do get the passport, be sure to file papers with the state department indicating that the child is not to be issued a replacement passport without your permission. Note that even if you have their US passport, if the other parent retains their foriegn citizenship, a foriegn passport can likely be obtained for the children WITHOUT YOUR CONSENT. Depends on the country of course. The state department has a working relationship with some embassies such that they can request that no passport be issued for the child. But again, it's hit and miss. Talk to the state dept.
If either of them files for divorce, in many states this automatically puts in place a restraining order that restricts either parent from removing the child from the state. This gives you some leverage with the state department and the foreign government. If no restraining order or such is violated, you'll have a hard time getting anybody to do anything. Either parent can legally take the kid anywhere.
Also check to see if the foreign country in question is a signatory to the Hauge Convention on Civil Aspects of International Child Abduction. If they are, you have much less to worry about, those governments have pledged to work together to resolve these situations, and respect each others civil orders, etc.
I'll get you some links.
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