[Solved] Advice needed

Hi all,

Apologies if this post is not in the right category. I am new here so wasn't sure.

Anyway I need some advice as I am not sure how to proceed, what the best thing to do is, and how to help my children. Apologies if the story below is long winded but I feel it is needed so I can connect with you guys and get the best possible advice.

I have 3 children with my separated (soon to be ex) wife. 3 years ago we separated as there was way too much arguing between us and it was escalating in front of the children so we both decided to do that. However 2 months later I was accused of walking out, not caring, giving up, even though we had both agreed to do this. At this point we had both decided to co-parent the children and turning it into a 50% - 50% situation. We had put a deal between each other in place that if we met someone, or started to have a relationship with someone, that it would be 6 months before we could introduce them to the children. I met someone, who I have been with since, and we have a very good situation and a very good home life, she has 2 children as well (very brady bunch!). About a year and a half ago, my ex started having issues with my oldest, who is 7 now, shouting, screaming, saying he hates her, then crying when she wasn't there or at work. Real separation anxiety and a host of other issues. My ex was telling me all of this, and I suggested going to see a child phsychologist, just us to see how we could both work together, she refused. Therefore I went on my own and got some good stuff from the professional. My ex then decided that my son needed to go and see someone as well, which I didn't agree to because how can she expect him to go when she wont go herself. Anyway he started seeing the Well-being specialist at school and hopefully that helped the situation. This behaviour continued to carry on between them, and then my ex decided what she needed to do was attend AA has her drinking was getting out of control. This happened 8 months ago, and I have been supportive of this, however her meetings and her dedication to the program has been put first and I felt that she was growing further apart from the children and not doing enough with them. I should mention that when the children are with her, they have a nanny, so she is able to leave them and get on with her things. 8 weeks ago, the children mentioned to me that they had met mummys new boyfriend (don't forget our agreement), this was the second time that this has happened. I asked her when she was going to tell me about this, obvious excuse etc etc. No matter, fine we move forward. A week ago, she told me that she was moving into her new boyfriends flat and that we should go to a one week on - one week off system with the children. I didn't really have many concerns about that but I did say that she should think about not moving so fast and rushing in to decisions which obviously benefit her but not necessarily her children. No response. My partner and I had them last week, and all was great. No dramas, no fights, no issues, great chats with the kids explaining to them the setup and trying to make it as smooth as possible. Now the kids are back there this week.

Day 1 with mummy: I get a message in the morning how they have had a major drama with my daughter (aged 5) dropping them to school and how she didn't want to go into the school with ** (mummys boyfriend) and mummy couldn't get out of the car because it was raining and she didn't want to get her hair wet.

Day 2 with mummy: I get a message in the morning and I think the only way to do this is type the message word for word so see below:
'Hi, can you please have our son tonight? He punched me twice very violently in the back on the way to school. Reason: Daughters water bottle leaked in the boot of (boyfriend) car onto his bag. He went crazy. He called her stupid and then threw his bag on the sand which then got covered in sand as it was very wet at the bottom. Then he refused to let me carry it as he wanted to remove the sand and we were running late. He stayed on the pavement cleaning the sand off with a stick whilst we kept walking. When he ran up to catch up he punched me in the back. I told him off for this and then he walked out the school gates again. We kept walking. He then ran up to me again and punched me harder in the back in front of other parents in the reception area at the front of the school. He then refused to go to his class. We were late so the teacher took him. I told him he's grounded and not going night swimming at *. He said he hates me and ***. So hes going to you tonight'

So before I replied to her, I decided to speak to the Nanny and ask her what happened this morning for him to be in such a mood. She told me the kids got ready, had breakfast and then were playing outside whilst the ex was getting ready. Apparently she was taking a long time, so they ended up running late. My son ran inside to get something, and then when he was getting in the car, my ex started yelling at him. The nanny then continued to tell me how they get smacked in public places, yelled at, and all the rest.

And now I don't know what to do.

Do I accept to take my son tonight, but what is that teaching him? Do I suggest to her to come with me to speak to a professional? Do I take my son off of her full time? Do I seek legal action? I am really at a loss of what the right and next step is?

Any ideas please or advise.