If I, an American, was in France and found a lost kid, I'd call the police. I wouldn't know anything about their social services or what their protocol was. Is the emergency number in France 999 like in Britain?
It would be really weird because I speak almost no French. "Er, parlez-vous English? No? Hm. La politz, um, por favor. No... politz sil vous ple. Je discover un bebe. Alorz!"
Probably incomprehensible, poor dispatcher!
If I was here at home, I'd still probably call the police because I don't know the phone number of social services while I do know 911 by heart. I wouldn't want to put the kid in my car and drive hir anywhere, unless we were really out in the boonies with no cell reception.