We need advise about our grandson who is 3yr old. His mother…

We need advise about our grandson who is 3yr old. His mother is our daughter, she also has two children from and previous relationship they are 10yrs and 8yrs old. She was with his (3yr olds) father for about 7yrs during this time we didn't realise he was violent and very controlling towards her.We live in France.
They spilt up after he attacked her (18mths ago) it was recorded as I had to take her to the hospital to make sure she had no broken ribs so she could fly back to France with me.
Since they split up he's attached her in her new home she set up with the children and this happened infront of the other children. She has never once stopped him having contact with his son. She has a lady from womens aid to help her as the police were taking him to court. Things started to get worrying when the son started to show violance towards our daughter and things he was saying was fightneing things a three year old shouldn't even know about, chainsaws.
I have the children every 3 weeks during the school holidays in the summer. After this visit the three children asked our daughter if they could move over, obviously the dad of the oldest ones weren't happy and expressed concern but our daughter told him its their decision not hers and after talking to the two of them he agreed, but as we explained to him very time we come over they will be with us and he has full access to the children so if they go on holidays he can take them. At the moment he only sees them once a fortnight due to work commitments. The two children are bright and well behaved.
With the youngest boy, due to the fact he was showing signs of upset and asking to stay with his dad it was decided between them that he would stay with him for a few days to see if he was happier there.
Well, after his weekend of having him he stayed so, we can over for a visit we asked if we could see him (our daughter has a new partner) the dad said he's upset and don't want to come down due to the partner being there. So we didn't get to see him, but our daughter went up the fathers house to see him. (This was a Monday) . We went for a meal to meet the parents of the new partner as they all moving over to France. Told the dad would not be up to see him he phoned her all the time to say the boys upset as he wants to see her (Tuesday). On the Wednesday, our daughter phoned him and asked if she could pick him up from school and drop him off to him later to spend as much time with him before she leaves at the end of October. Well, he turned round and told her she couldn't and that he is reporting them to social services as the new partner is apparently "hitting" him. He turned up at my mothers sheltered home complex at 13:30 and kicked right off, it's not the first time, I had to physically push him off the premisses he was very abusive to me and my hysband and he went to the school to remove the boy. He didn't manage to take him as the womensaid lady was in contact with the head of the school and explained the issues that was going on. So, we managed to see him that afternoon. As soon as he came in he ran up to me and hugged my legs and asked if he was allowed to come to France with me. That night at home he was kissing his bed and saying he loves his bed.
Last week they came back to France to look for jobs and to get the children into school we had to bring the youngest due to the father, but he new about it.
Well, he's been txting and being abusive towards our daughter which is kind of understandable but, I do feel it's his own doing she was a willing for him to stay along as she had access too. The father has snatched him now from outside her house and won't let her have any access to him he's stopped him going to school. She phoned the police they didn't do anything. Then she called social services no complaint has been made, saw her solicitor again and she's wrote a letter asking him to return him but, no he hasn't.
I worry what this guy is telling him as he has made threats towards the new partner that he will cut him up into pieces (the chainsaw) and we have the boy saying this also. He is not a very nice person you only have to mention his name and the reaction is not good. The family is not good his brother has just gone down for six yrs for trying to smuggle £200.000 worth of cocaine into South Wales. All his other partners he was violent towards the oldest daughter would only go up the house if he wasn't there to see my daughter and her brother. She was going as a witness for her if it went to court but some reason he got off with it. Please can you give us some advise how to get him back.