Children are sending these text messages to one another not knowing the consequences of their actions. I don't know how old your children are but child pornography is serious and can have both state and federal implications.
It's not good that these images made their way to the internet because these images are going to be flipped and traded amongst people seeking child pornography and every time these images are viewed your child is being victimized.
You need to speak with a lawyer that specializes in sex crimes and federal child pornography to determine if the images you have described would be considered pre-pubecent images.
You need to speak with a prosecutor. You may be able to press charges against whoever sent the photos out. In addition, if the people in the photos were under 18, there may be an issue with child pornography. For anything further, talk to a local attorney.
[This communication is intended as general information and not specific legal advice, and this communication does not create an attorney-client relationship.]