Name changed for obvious reasons on this one. I split from my ex the beginning of last year he was having the youngest two dc every other weekend (oldest refuses to go). We split due to him being lazy, didn't treat me like a human and jealous of his own children to the point he did everything to keep them away from me. It took me a decade to finally get the courage to end it.
Anyway he now lives at his mum's and has since cut down seeing them to whenever he has free time as he changed jobs but they no longer stay. I kind of feel like this is a form of control as he never gives any idea when he will be off but that's another story.
The problem I have is that recently two dc have been coming home telling me that he is nasty to middle child. I asked what they meant and they said that he hits him and lies about it. I asked them seperatly what they mean and it seems that he hits ds and when they tell his mum he outright denies it. They also said it used to happen while I was working. I asked oldest about it and she said that she used to walk in to ds crying and saying dad had hit him and when she questioned him he would say that they were playing and ds fell over. I've even had youngest crying asking for him to come back here as he is nicer when he lived with us. It's heart breaking.
I questioned ex who just said I can't believe they would say such horrible things about me and how it hurts him so much that they would lie like that. (Always about him). I use his mum for childcare and now I don't know what to do. One of the children said last week that they are glad he's not in when they were going there so he can't be mean to d's and she said "Don't talk rubbish he doesn't do that" then went on to say how she used to be always snaking her kids and still would if they wernt bigger than her 
Now I don't know what to do I feel like I should tell someone but family members say I should just keep them away as if I report it they will be taken from me as I have smacked them once in the past (once and I apologised to them for losing my temper and would never deny it). Plus I also feel the fact I missed loads of signs my ds anger after coming back from there, the fact he wet himself until he was 8 and when we went to the specialist ex told them he drank loads of fizzy drinks (He didn't) so they said his wetting was down to that. His lack of confidence. I feel like I've failed him so much.