I think I'd do as nowcemsi suggests. Don't bad mouth her or the situation but you could just tell it like it is. 'Daddy decided he would rather spend time with the other lady than Mummy which is why he left and that's why Mummy doesn't really like the other lady'. The children will make their own minds up eventually but you don't have to pretend to like her just explain in whatever age appropriate way why you aren't happy with her or him.