MM, on the one hand you say that you expect her to chip in and help if one of the kids is sick or something, and that you hope in time she might love your children, but on the other hand you say you don't need to let her know anything as you only need to maintain a relationship with your ex.
Can you see how contradictory these positions are? I know it's very easy to say and you must be going through agony knowing that that this bitch is playing happy families with your kids, and frankly I'd want to kill her, but you can't have it both ways. If she is part of your ex's life, then she's part of your children's lives for the time being, like it or loathe it, and that has to be acknowledged, not for her sake, but for yours and your children's.
Please don't think I'm minimising how tough it is and how unfair it is - I think it's bloody awful that mothers are forced to eat this shit when break-ups like this happen - but tbh the less you are able to distance yourself from it, the more like eating shit it will feel. And I know it's tough because it's still so raw - five years down the line it won't feel so bad, as long as you play it right now. But tenalady, however bluntly she's phrased it, is right. If you can't at least pretend to let go at the moment, you may be starting on a long career of years of hostility, anger, niggling, wind-ups, etc. about the children. And that will not do them or you any good - you deserve better than that, not to be stuck in that awful, self-destructive rut.
Having said that, I don't really know how you can "let go" as tenalady puts it, at this time. I don't think it's possible to let go of such a bloody major betrayal so soon, but you can pretend to. This is one area of life where denial and pretence are probably necessary as a short-term strategy. I won't be so crass as to tell you to prioritise your children's needs - I know you'll do that anyway - but please prioritise your own needs too. And your long-term need is to find some kind of way where emotionally, you can deal with the fact that your children are going regularly into the lair of your enemy. If you start off with the negativity now, five years down the line, instead of having emotionally moved on, you will still be embroiled in the heartache, betrayal etc. and it will still feel the same each weekend they go there. And that is just a horrible prospect for you, please try and avoid it for your own sake as well as your children's.