Had go-kart accident with a friends child who broke a leg, not on our property, his mom said yes r we liable?

We had a terrible accident with the little boy next door, 5, it has been the most awful thing to ever happen. Our kids rode with my hubby all day and no problems. I talked my hubby into letting the little neighbor boy go for a short ride, and tradgically some how his leg slid between the bars on the bottom and broke his femur. We have helped with hundreds of dollars and let them use our vehicle to go to Hosp and appts as theirs is not reliable for long distance. Anyway we have not had any problems he is healing well. And his mom and dad have been very understanding. It didnt happen on our property and his mother gave him permission, i dont think they r gonna sue us, we have been there to help through everything but im curious if in they would have a case against us? Just need to know, in Michigan. Thank you for any info, like i said I have 3 kids and this has been the worst thing our family has ever had to go through. We plan to continue to help with $ and everything else but i just need to know if they would have a case. Also it was a store manufactured gokart, not that it matters really.