
Pedigree do tighten up tight...well mine do. My fried who it quite small and is very skinny wear's one that would cut my circulation off ;)
I do not know if any other ones that will change size.
You could either adapt it by getting your parents to put in stechy velcro and take out the elastic in there already or use just a rubber band and don't use an arm band untill you can fit into one! ;)
Either that or use a good old fashiond ringclip on your arm (used to be all the fashion about 10 years ago!)