"Oh, poor children!"
Translation:¡Ay, pobres niños!
Often, pobre after the noun means without money; before, it means deserving of pity.
But there's no context given. This person could just as well be talking about children without money. "Niños pobres" should still be accepted.
Actually in this interjection it is unlikely that one would use pobre(s) after niño(s) for it is a sympathy expression that has nothing to do with wealth.
To refer to their wealth the phrase would be something like ¡Ay, que niño(s) tan pobre(s)!