We've faced this several times in the past and it never gets easier. Knowing what their biological mom (a.k.a. the Momster) is like, I can't imagine having to spend a week with her. Since my step-daughter is in trouble for participating in the school talent show, both she and her younger brother are being punished at their mom's house. Somehow, their older brother (age 15) escapes this. It might be that he's his mom's biggest fan. He's constantly covering up for her and defending her. Just this week, the kids were laughing about how their half-brother (age 3) now says the "f" word. We asked how on Earth he learned that word at age 3, and my step-daughter and youngest step-son both said that their mom uses that word a lot. Not that this is a big deal to us - she can talk however she'd like in her home and among her children. However, my oldest step-son immediately jumped in to "defend" his mom, saying that she never uses that word and he accused his siblings of lying. It's fairly obvious who was lying in this situation, and he's not saving his mom any shame in trying to cover for her.
Anyway, the younger two certainly dreaded returning to the Momster's for her custodial week. In a way (and this is purely selfish), I kind of relish in the fact that she's such a bad mom. They're not my kids, and I have to try much much harder to gain their respect and approval... so when she is blatantly horrible to them, I silently thank her for making my job just a bit easier.
Not that it's easy to deal with emotionally distraught kids or know that you're sending them to a home in which they are merely "things" for her to toy with. It's certainly hard to know that they're unhappy this week, and I don't want them to experience unpleasantness for one minute.
However, it is reassuring to know that their mom slips up from time to time, and shows them her true colors. She is a nasty woman, and every once in a while they get a taste of just how nasty she can be. During those times when she is publicly campaigning for the "Mom of the Year" title - which she often is - it's helpful to think back to times like Sunday and realize she's not fooling those who matter most.