I tell my children to ask one of the following people for help:
Policeman or policewoman or security
Uninformed worker or shop worker
A women with children.

My friend however thinks that this is ridiculous and that children should speak to no one if they are lost and stay in the same place and wait for someone to approach them and offer help and hope that that their parents find them.

What do you tell your children to do?

Hi, also get your kids to memorise your mobile phone number. This can take some time, so start early, make it sing-song and eventually they'll get there. It took ages with my dyslexic daughter, but now she'll never forget it. I've never needed it, but my friend's 7 y.o. got lost and this ensured they were reunited very quickly, as she'd told him to approach the same list of people as Emily said above. Good luck!