OP said kid was running around, not looking like he was forced to do something... must have been a newbie at the photolab, who already thinks people who shoot film are strange, so when he sees the naked kid he freaks....

If the op, took a whole roll of the naked kid, somethings weird, if the op took a photo, where the kid looked like he was in distress, again weird... family photos, and naked kid running around, funny.

to be blatant, as i get older more friends are having kids, and many of them are from el paso, tx (90-100 degree weather in the summer)... so amongst photos of food being grilled, family,etc... there's a naked kid here and there... no one thinks it's weird, because it's in CONTEXT.

so op was singled out for a reason he is unaware of or omitted... or the photolab people at that specific place are 'sheep' to put it nicely.