But, "risk" of what damage? People who would do that could do it to any one of the (literally) millions of pictures online. Just Google, for instance, "[fill in any age] year girl swimsuit" then click "images" at the top ... you can find a dizzying array of family or news photos out there. Moreover, these people don't need to Photoshop your child's face onto any other image, they have an ample supply and your child's visage is hardly likely to prompt a special compulsion.
The photoshop abuses are not just a matter of sexual compulsions. Just Google "I can count to potato!" and you'll see why an above poster's cousin removed her special-needs child's photo from the web (safe for work, but will enrage your sense of moral decency). Or, consider the deceased bullying victim, Rehtaeh Parsons', photo being used on Facebook in an advertisement for online dating
just a few months after hear death.
I don't actually think it's dangerous
to post photos, in the sense that there is a risk of abduction or assault. Statistically, North America has it's lowest violent crime rates than in the past 30 years! However, there will always be some risk of misuse or misrepresentation possible.
My job requires working with stock photography. More and more , it seems that companies have no clue at all about how copyright works and/or that you can't just use some random person's photo in an advertisement. There are important protocols to be followed and waivers that must be signed before you use someone's likeness, other wise you end up with a lawsuit when someone's face was used in an "I'm positive!" HIV awareness campaign without her knowledge. (Photographer messed up the release.)
But generally, I would respond to paranoid busybodies with a "So kind of you to take an interest" and then bean dip to something else. They can deal with their own fears as it pertains to them, and you can go ahead with what you feel are reasonable risk tolerances.