I'm a very long-time lurker but this is the first time I've felt the need to post on AIBU. This is a bit long, sorry, but I wanted to give a reasonable picture.
Last week DH and I went to visit his sister and her 2 year old DS. We don't have children of our own but we are both used to spending time with small children.
SIL's DS has taken quite a long time to reach various developmental milestones. For example, until very recently he couldn't speak and is only just beginning to say a couple of words. Her family has been quite worried about this but SIL hasn't been concerned.
When we arrived, I expressed how good it was to see her DS saying a couple of words. She responded by saying that she'd received an appointment in the post for a review of his developmental stages but that he couldn't do half of the things on the list so wasn't going and binned it! 
Later, we went for a walk. DH told his sister that we couldn't be long because we needed to get going soon (he has a broken wrist and it's a two hour journey home). After about half an hour DH said that we really should be getting back now, but his sister ignored this and went off in the distance with her DS.
She then decided that her DS needed feeding and proceeded to very slowly feed him two packets of food. At this point, DH was a bit firmer and said that we really must get on because he was very tired and his wrist was hurting.
Eventually we started to make our way towards the park exit... Except, there was a wall that SIL's DS wanted to walk on. We figured it would only take a minute so smiled along. But it didn't take a minute... we spent about 15 minutes standing there while he walked up and down the logs very slowly, with SIL making no effort to encourage him off. DH walked up to him and gently tried to grab his hand, at which point his sister pounced on him and told him to leave him and let him play on the wall.
DH was a bit irritated now and said, 'Look, we need to go.' SIL snapped at him that it was her DS's time and their routine. 
Finally we managed to move away from the wall, only to come to the car park. SIL's DS pointed to some letters on the ground (as in, SLOW, EXIT etc.). SIL then spelt out a word as he pointed at each letter on the ground. We were open-mouthed.
But, what really shocked me is that SIL's DS was in the path of a car reversing, so DH ran towards him to grab his hand and pull him out the way. SIL was furious and told DH to get off him. 
We've come away feeling really shocked by all of this. I don't think this is a good way to be around children?