I've never tried contracts but last year we had a similar problem with girls disappearing when duties were being done (we had a lot of new girls and I had unfortunately made an assumption that some of the girls had done duties before - we had some transfers).
For the next time, we made up a list of what had to be done, did inspections and gave stickers (on a 3 point system) for the duty patrol. If one girl didn't participate in their duty group then they automatically lost a sticker. The stickers were posted by the duty list. At the end of camp, the group with the most stickers got a little prize (stress little- like a little hat thing or whatever or first choice of whatever we were giving to everyone).
By our final camp the majority of girls were doing their duties without having to be asked (or only asking once) - peer pressure works again. Originally I didn't like the "bribing" - I never had to do that before - but it has helped a lot and the girls have realized that if they do their duties quickly and correctly the first time, we can get on with the more fun things planned. I guess the stickers also show that they are doing a good job (visually). I'm hoping that this sticks for next year. This may be a bit juvenille for pathfinders though.
Contracts might be a bit much, particularly if it was only one kid but I agree that singling her out wouldn't be a good idea. Some of them may take it personally - it depends on what your particular group is like. A discussion on what the Pathfinders expect to have to do at camp is a good idea. Since she is the youngest - the older ones should be setting the tone on what is expected. Consequences should also be determined. I think parents should be informed of the consequences or contracts so they don't have any surprises if there is a problem. Just curious on what would you do if the contract was broken? It would solve any parent-leader disputes if you wanted to refuse to take the girl to camp or send her home early due to behaviour problems.
An interesting problem, that's for sure.