Should I allow my soon-to-be ex-wife to take my children to Sweden?

The pre-hearing trial for our divorce is on 3/31. It's at least a 4 hour session. It's where all the issues of our divorce, a divorce she initiated, are to be resolved: custody, ets, alimony, etc. I learned from my daughter today that their mother is planning to take them to Sweden during spring break, which coincidentally begins 3/31. That's a Monday, so presumably they'd leave on the Friday evening or Saturday before the 31st. What's wrong with this picture? She has a best friend of long-standing who lives in Sweden, and a former boyfriend lives there as well, I think. We've been married almost 18 years. The kids are 15 and 11. Would she come back? What are the immigration policies in Sweden? Can we have some clause in our divorce decree that prohibits either one of us from taking the children out of the country without the other parent's permission? I really don't want to deprive my children of the opportunity to go to Europe, but this doesn't p the smell test.