Try and make sure it never happens again... If he is scared of the images make sure he understands "real" vs "pretend"... Talk about ketchup bottles and actors getting up to act in something different next week, it's not real.
If you find it hard to take on mother in law and her childminding activities, you could just outright ban him from having the IPad when she is there. If he still gets it when you are around it ought not to be too strict for him.
I sympathise. I have a relative who was allowed by an elderly relative to watch all kinds of gory sh*t... Means he now has a very bloodthirsty imagination and seems inured to violence. As far as I am able I don't let him anywhere near 18 rated dramas. I might describe a horror scene to him but don't let him watch horror.
Im completely new to this but am really concerned after returning home today and my mother in law telling me that she found my son viewing very unsuitable video clips on youtube of adult horror, I watched everything that he had seen by viewing the history and am devasted that he has seen images way beyond his years. We as a family have allowed him ipad access in the past but this is controlled by us when we are at home and as my mother in law is very out of touch with modern technology and my 4 year old is a dab hand at using it I dont think she appreciated the risk and busied herself with my youngest and whilst he had free rein he very quickly went from viewing spiderman cartoons to clips from paranoramal activity and blood soaked heads and all sorts of grusome and frankly what I imagine terrifying images, obviously its too soon to know if it will disturb his nighttimes, I spoke to him about about what he had seen and other than saying it had frightened him and that he knew it wasnt very nice he was pretty dismissive. Can anyone advise how best to handle this please?