@Sruf They're sure to explain next chapter.
If I had to guess, I'd say they're assuming that breaking out would provoke the enemy. If it can kidnap them that easily, then it could also just kill them if they cause too much trouble. They're probably planning to break out when Will and his dad arrive, so the enemy is too distracted to notice the escape, and they can attack from 2 sides. They are already at the enemy base, after all.
That's just a guess, though.