Hi,
I have some experience of safeguarding procedures. I can see that this is a real problem for you and the people who should be helping are not.
Although morally if you and your DH complain directly to SS about this, SS should do something (as pp have said, what the ex wife's partner has done is illegal and is actually assault), I think as you say it would possibly end up wtih the ex wife and partner saying you guys are making malicious claims. And might well end up in lengthy court battles etc etc.
If instead, the children can be persuaded to speak to someone at school (have a look on their website to see who is the named lead for safeguarding) I think it might make the process simpler. Do they have any marks on their bodies at the moment caused by this awful partner? If they do it makes it easier to get things changed quickly, because soc services would have to find a place of safety for the children which would obviously end up being you guys. If you haven't made the initial complaint it makes it easier for the SS to place the children with you without your ex wife and partner being able to fight it.
I am absolutely shocked at the SENco implying she wants nothing to do wiht it. Words fail me. Just awful. Safeguarding is everyone's responsibility.
Anyhow so I think what I would do if I were you would be to find out who school's safeguarding lead is, then on Monday morning (tomorrow) ring the school and say you need an appointment with this person this very morning. If the safeguarding lead isn't available, the school should have a 2nd in command designated safeguarding person so they shouldn't be able to fob you off to the next day. Or just turn up there with the kids and say you need to see someone face to face about safeguarding. I'd suggest talking through with the kids before you go that this is the plan and you know how hard it is but it is incredibly important. You can look up the school's safeguarding policy online and read it before you go so you're armed with what they SHOULD do from that meeting.
Then go to that appointment - both you and your DH with the children and at the appointment, say you have concerns about what the children have said. Then hopefully the children can say these concerns to the teacher with you present at the meeting supporting them, but it ends up being logged with soc serv as "kid says to teachers they have been kicked" rather than "husband in legal batte with ex wife has a complaint about her partner".
Good luck, sounds a horrendous situation.