I'm sorry that so many of the grans on here feel it necessary to tell you that your grandchild being bitten is none of your business OP. Naturally if you're a close and loving grandmother, the thought of anything, or anyone harming your grandchild is going to be upsetting. Personally I can totally understand you being hopping mad about it, I would be too.
As for those that have told the OP that it's none of her business, not to get involved, etc., I don't think for one moment that she is planning on going to the school, or approaching the parents herself, merely hoping to support her child and grandchild. So no need for the snide comments, it's upsetting and unnecessary.