As always, I think preparation is the key. I looked at some technical solutions and some commonsense ideas that could help:
1. Lost child alarms. These are those fantastic paired gadgets for kids and adults to wear, that stay quiet as long as you and your offspring are within a certain distance of each other, but as soon as the distance becomes too great, both gadget emit a noise to alert all parties to the fact. This is particularly useful, as it seems that in a great many lost-child cases, the child in question doesn't know he's lost for a long long time.
I once watched my 18-month old daughter arch away from her mum, and decided to follow her unseen, She wasn't a very strong walker, but she managed to walk continuously in a straight line for about 2 minutes with not a single look back.
In the end I decided not to buy one of these, but there are many interesting options on the market.
Levana CT1000 Digital Wireless Child Tracker Wrist Watch and Kid Finder/Locator
Mommy I'm Here cl-103br Child Locator, Brown
2. Mobile phone. In terms of cost, this is the nuclear option, and of course it is only reasonable for older children, but then you can really not worry about losing track of each other, and even plan some short alone-time periods to build up their sense of independence.
3. Whistle. Simple, yet effective. If nothing else, this attracts lots of helpers, and can double as a bully/dog alarm.
4. Mobile phone number on arm. Yes, I really do write my mobile number in big bold numbers on my child's arm on special occasions. If you can stand the angst of the similarity to Auschwitz tattoos, this is a great idea. We gave our kids standard instructions at the time of "tattooing", that if they realize they are lost (This often takes quite a lot longer than you might expect) they need to show their arm to a suitable adult. Excuse the profiling here, but this is for us, is a nice-looking lady, preferably with kids in tow, or someone in a uniform behind a counter.We even had some friends who, because they frequented concert festivals a lot, had T-shirts printed for their kids, with photographs of the parents on!

