5 yr old DS LIKES having a "festival wristband" with my phone number on if we go to big busy events. If I haven't got a band to hand I write my number on his arm. We also go through the drill with him about who to go to ( steward/ stalholder etc) and what to show them to if he can't find HIS grown ups. Actually , this summer should be easier as speech therapy over the winter has resulted in his speech being understandable. Last year it was hit and miss if A.N.Other adult would have understood his speech. So closer to toddler than a typical 4 yr old in some ways. I've used bands ever since he was mainly out of the pushchair at events.
I was given a bunch of single use medic alert bands by the Lions club. They dish them out at the county show and similar.