I have had lots of practice at this
Firstly, yes it is far far better that DC get on with her than if they don't. It may not feel like that now, but having had both extremes, I can assure you, it's better they get on with her the nasty bitch
If you try to slag her off in any way the DC will side against you. I know it is awful and unfair but they will. SO it isn't even an option.
You have to make it a joke - here are some real life samples of the lovely things my DC have come out with over the years:
"OW lasagne is much nicer than yours mummy." Is it? That's probably because I don't like lasagne much and you're always better at making fod you like yourself.
"You should ask OW for her gammon recipe. She made it for us and it was really really nice." OK - get her to pass the recipe on, that's a great idea. Then I made it and they said it was better than hers 
"Mummy, OW is really slim, she goes running every day, do you think you should go running?" Mummy isn't built for running darling.
"OW is soooo healthy. She doesn't drink alcohol at all." That's nice dear, I expect Daddy likes that, and then she can drive him everywhere.
Various other things - all answered with variations of "good for her."
It is hard but having been a small child on the other side of this myself, it's a tricky thing to navigate emotionally. I hated my mother for slagging off my stepmother who was absolutely charming and kind to me.
Smile and wave. It's all you can do.