How old are the kids?

+1 on adding a comfort item unique to each girl that can ease their mind in a stressful situation.

Add a laminated contact list of people & #s for the girls that they can call or if a someone else finds the bag, allows them to contact the parents to notify them. Keep the bag as light as possible because kids aren't very tolerant of carrying extra weight and are likely to put it down and leave it if it is a burden.
Stitch their name or label each item or add a little flower stick on to decorate things. Situation example: if they are lost/separated and use one of the items and then leave it behind, it helps trackers to know that it was theirs. Also, having their name on it settles arguements between the girls as to what belongs to whom! It gives them a sense of "this belongs to me, maybe I should take care of this!"