The thing is, you don't really want all the PCs dying in jail, so why not have the Eladrin be their escape plan? You want them to get out anyway, why not have it as that PCs time to shine?
There is a big difference between getting out of a cell and being free. They will have to wait for the right moment to teleport out, find the keys to release their friends, maybe fight off a guard, sneak past a guard dog etc. All the things you would expect from a jailbreak scenario, except now it is the Eladrin doing it, rather than the party theif.
In my games, few people know the difference between Eladrin and Elves. Most guards will never meet one in their entire life. It is a good assumption that they will be utterly taken by suprise by Feystep. I might have them blindfold all the prisoners, but not because they know about Feystep. Or maybe the old-timer guard knows "Yous gotz to blindfold those elf witches or they charmz ya and killz ya in ya sleep", so they have the right idea, but for the wrong reasons.
If the Eladrin does not pick their timing right, or fails the challenge to release the others, eventually a more educated guard would figure out that they can teleport and take appropriate precautions. I would suggest lots of silly supestitious 'remedies' to bind the Eladrin, but in doing so they also find something that works eg. Tie garlic cloves and mandrake around their head (which just happens to block line of sight).