Have I got this right: you got a new boyfriend last year (so some time in 2023?)
His ex wife has a sex offender boyfriend so you told your boyfriend to ring social services, which he did.
Then his ex wife accused your children of bullying, but said it was YOU rather than your children who weren't safe near the children?
You opted out of the drama.
His kids are now on a Child In Need plan.
If you had another go at the relationship you would not go near his house or kids due to their "High Conflict Birth Mother" (good luck using a term like that on here in respect of the mother of the children of a boyfriend you saw for a few months last year).
Do the children live with his ex-wife?
If so, and it's her parenting that has caused them to be on a CIN why are they not with him full time and having contact with her?
Were your children bullying his?
Personally I would leave this drama-soaked nightmare a long way behind and find someone stable. If the children are on a CIN then in theory social services might be interested in checking the background of any new partner but if you are never seen then it's unlikely to be a priority, unless it's relevant to the reasons they are on a CIN.