My eldest son has been to various BIG rugby and football matches over the years; he reliably informs me it was really uncomfortable having phone numbers written on a piece of paper and tucked into his sock.
I love these emergency contact information transfers - my ex-husband has some for our youngest, and we both think they're fabulous.
Basically they have your contact phone number on them and transfer on to your child's skin, the way normal temporary transfers/tatoos do.
Your child can choose the design they'd like, and you can choose what you want it to say: Please call mum, call my dad! or whatever you like.
There are also ones for allergies too - which I think is a VERY good idea!
This is one of those simple ideas that makes you wonder why YOU didn't think of it sooner *grin*
They cost £6.50 for a sheet of 12 and delivery is £2 on top of that.