To be honest this is why I would never approach a child I don't know who looks lost, perhaps I might nowadays if I had my own toddler in tow as people react very differently in that context. I'm 6ft beardy and to some might look a little frightening.
There have been a handful of occasions where I've seen little ones gotten lost. My usual MO is I stop what I am doing and just wait. I keep an eye on the child and wait for the frantic mother to come into view, once reunited and safe and sound no need to do much else.
Only times it's happened to me have been in shopping centres, I guess I'd weigh it up differently if it was near a road or there was some other environmental hazard, but generally I don't see the need to get immediately involved. Only times it has happened I've been on my own, if I had someone with me I'd send them to go find a member of staff to make an announcement, and just stay nearby to make sure the child stayed safe.