Chad, I haven't had a chance to read this thread in full and therefore it's very likely the others have already said what I'm about to say. I will not have to point out that there are no restrictions placed upon photography in a public place, nor are there any separate laws for minors, nor does it matter if they are the main subject of the photo or unidentifiable elements (as they are in your picture). Therefore you can publish this photograph how you wish and you can even sell it for newsworthy, educational, or artistic purposes. The only exceptions would be if you were portraying them in a derogatory manner, or if it was an invasion of privacy (like if you'd done a sneaky shot of somebody in a public toilet) or using the photograph commercially (this would mean using it to advertise a product or service, in which case you would need their permission). Therefore if you are using it to promote yourself then their opinion has some merit, but if this represents your personal work and is distinct from your commercial work then they do not have any grounds to insist that you take down the photograph. The fact the subjects are not even recognisable makes their complaint seem even more strange.
I would respond very politely (I think it's important to acknowledge anything like this) and ask why they feel there is an problem (because there is certainly no legal issue).
However, even though you have done nothing wrong, you can still decide whether or not you intend to remove the image simply as a matter of courtesy (this can often be the best option, particularly if the image has no value to you). But if you do this, you should also make it very clear at the same time that you are under no obligation to, and it is a favour. I have had this happen to me before, and I understand how frustrating it can be.
EDIT: just read the bit about Facebook. Sigh. An ignorant and misguided response, unfortunately typical of 'social networking'. I would agree with previous advice to send a message to the administrator of that particular page, explaining the position. However I suspect they are objecting to the fact you did not remove the picture immediately, as per their demand.
