Hi Helchi,
You and your ex will always be your kids’ mum & dad. Your ex’s new wife has no legal say in their lives.
On the days when your children are at their dads, he can leave them with anyone he wants to as long as they are safe. So he might leave them with her while he plays football. And on those days he will obviously expect them all to treat each other with respect.
But he needs to tell them he has remarried. That isn’t your job.
How old are your children? Have you talked to them about their dad being with someone new. I think I’d ask how they all get on together. Make it easier for them to talk about it.