As has been said, repairs to childrens glasses are free, unless, you have chosen to pay for private things on top. We pay for my sons lenses as he has a strong prescription and we pay for thinned down lenses, these cost about £50. That means, because we have chosen to pay in the firts place, if he damages the lenses we have to pay to replace them - which as my son scratched them badly last week, we have just had to do, however, the cost to replace the one lens was £35 and would mean he was without the glasses for for 10 days. However, he is covered to get a new pair of frames even though they weren't damaged, so the cost of a full new pair was £47 as we dont have to pay for the replacement frames. Looks like she is at it to me. most opticians have plenty of choice of nice frames for kids, either free or for a nominal charge (£10-20). Not worth paying more for a 10 year old.
it would be unlikely that the glasses and frames were broken so either it would be to replace the frame which should either be free or cheap or one or more lenses, which again should be free or cheap.
Apart from all that, she has to take some of the responsibility as her son started it. I would say go 50/50, but only of what I would say was a reasonable amont - say up to £50 so £25 max for you.