You can buy wrist bands from stuck on you but my boys find them uncomfortable and rip them off. Your children are a bit older so they might understand why they're there and leave them on.
Other options are writing it on a bit of cloth and safety pinning it on.
For somthing like the show or the footy I just write my mobile number on their forearm but it's not so effective internationally because of the IDD thing.
I saw some chidren with those plastic key-ring thingies with the paper for writing on around their necks on a flight last year.