If it was an accident I would call the parents and have them come over. They probably did not mean to so getting the cops invovled would be a little harsh. There is always work to do around the ranch and @ 35 dollars a bale and 5 dollars a hour looks like they owe you around 700 hours of labor. Keeps it off their record and they might learn something.
In my opinion, sueing the family or calling the cops would not be an option. That's the problem with Amearica today, everyone's sue happy! I would first talk with the parents and kids and tell them what I had in mind as repayment, which would be work, and I mean lots of it! If they diagreed, then I would "get a rope".
As far as the work goes, I would have them clean several hundred acres of mesquite with an ax and spray the roots with diesel and remedy. Then I would have them clean out any fields that might have rocks in them. Pick up the rocks, put in bucket, and haul to edge of field. They could also do other misc chores around the place, such as mowing the yard, trimming tress, clearing out fence lines, etc. After about a year of this labor, we could call it even. And on the last day of work, I would present them with some fireworks. ;-) ;-)
I agree with the whole work thing but you have to have their parents backing you. If they are the type of parents that don't give a rats *** then you might have to go the hard way and get the law involved. I wouldn't let it just go. I don't know about you but if we lost 90-100 bales we would be in a huge world of hurt since that is more then we usually feed in a year. I would not have the option of allowing the kids to work off their misdeed, I would have to have the hay replaced or I would have starving cows come winter time.
Boy, that must of been one heck of a fire, cause when we first bought our place, there was about 15 rolls of old hay left out here. Since we didnt have any cattle and none of the neighbors wanted it because it had weeds in it, I decided to burn it up! They made for some huge fireballs, and talk about smokey, man you couldnt see across the pasture when we first lit up each bale. These were pretty tightly wound bales and it took the better part of a day to burn them all!
Any chance their parents can replace your hay with some of their own? That and a summers worth of work along with help bailing the next few times might be all you can get out of them.