To those saying that the OP should have stopped her son playing in such a manner when she could see he was getting too excited, please bear with me while I talk as if I am the OP for a minute:
My DH's Mum, has helped us so much over the last 6 years with our ND son. She has been absolutely brilliant up until now, so I hope DMumsnetters that you can understand the shock that both I and my DH had when DS's Grandma lashed out and hit him on his back, after he hurt her - he bit her.
Of course I know that biting someone is a horrible thing for him to do, and sadly it is something he has done for a long time now, it happens on occassions when he gets too worked up, too excited. My DH and I have been working on this behaviour of his, (with advice ftom the appropriate quarters of course) and it happens a lot less now than it used to.
Our particular problem here is that our DS's DGM has known about this problem with our DS's behaviour for almost as long as we have, she knows what triggers him, and she knows that we do not believe in corporal punishment. But on the other hand we do know that not everyone reacts in the same way when they are shocked, especially if that shock includes being hurt. So although we were very shocked that she had behaved in such a way, and I have to say we were also very disappointed by her reaction when he bit her, we could see that maybe she couldn't help lashing out in surprise and pain. So if after we, and our DS apologised - it took a quite while for our DS to calm down enough from being hit by his DGM to be able to apologise to her - she had apologised to our DS for hitting him, and explained to him herself, that hurting other people was not an acceptable way for anyone to behave (except in self-defense if you are being attacked - but I am not sure if my DS is mature enough yet to understand that distinction), then I would not be writing this here.
Unfortunately, when speaking to her on her own, she did not show even one small bit of remorse over her reaction to our DS. My DH and I were shocked all over again, when not only did she not feel terrible about hurting our DS, but despite knowing (and us having just reiterated it to her) that we never use corporal punishment on our child - or anyone else's for that matter - and that we believe very strongly that purposely hurting a child is the worst way to try to teach them to not hurt anyone else, she told us that we can discipline him in whatever way we choose, but when she is looking after him, she will discipline him in whatever way she chooses.
What I think that @Maynot has not addressed yet (probably because of all the unreasonable and non supportive reactions she had had from too many Mumsnetters on this thread, so she is not thinking straight), is that she, and her DH did not intervene earlier when their son was getting too hyper, because after 6 years, they thought that Grandma would have it all in hand, and would know when and how to calm her DGS down.
It could have been happening just a few feet away from them, but surely even mums of NT children, know that all good, involved, parents need some time out to relax. We see every day on Mumsnet's pages how single Mums, who have no supportive network around them, can really struggle, but for many - I hope for most of us - having no family support at all, is something that we have never personally had to deal with, therefore we feel so very sorry for the Mums (and occassional Dads) who suffer from that lack of essential support.
I know that when my children were young, and my own DM was visiting, I could breathe a sigh of relief, have a cuppa with her in the same room as our children (I say our, because I considered my mum to be their second, and in all honestly, better Mum, than I was - I obviously don't expect most people to feel like that!), and I could zone out, and maybe daydream - my school said I was an excellent daydreamer when I was there - while my Mum looked after the children barely even a few feet away from me! I think that that is what probably happened to the OP and her DH, they just, quite naturally, zoned out.
Can all the Mumsnetters who have commented in this thread, but not been supportive of the OP, honestly say that when they have been in a room with their young child or children, and another responsible adult has been in the same room, interacting with their DC, that they (Mumsnetters) always stay on full alert? Lastly may I say that if you only ever give over responsibility of your DC to paid adults, and only outside of a home setting, then maybe you need to learn to relax, even for a little bit, or you are in danger of wearing yourselves out, and that really would be bad for you, and your children.