I agree with them.
I just want to highlight how you're doing an awesome job of making boundaries that involve you and your husband, as opposed to rules to dictate the relationship between your husband and his girlfriend. So you have your night, and she has hers. You're talking about how you feel about your night being invaded, as opposed to how she's behaving outside of her night.
From the way you said she came into your room uninvited and unannounced, I assume she lives with you? In that case, I would invest in a lock for your bedroom door. I also want to say a "Do Not Disturb" sign, but I'm hopeful that this can be resolved without going to those lengths. Either way, walking into a couple's room like that is extremely rude. She wouldn't do that if she were a houseguest staying with you for the weekend, and it's not her room, so what makes her think this is acceptable?
It's been said that one of the great things about polyamory is that it allows child care to be shared among more people, giving everyone a bit of a break now and then. This should not be confused with you and your husband acting like free babysitters. If you and your husband had chosen to spend the night out with your cell phones off, and she was too tired to take care of her own kid, then she'd have to figure it out on her own. It doesn't sound like it's his biological child, and I don't know enough about your history and arrangement to decide whether that would change anything.
Now to try and see things from her perspective. It would be difficult being the live-in girlfriend of a married man. She's trying to establish her role and position in the relationship. She may be intimidated by your closeness to your husband, and perhaps she believes (maybe unconsciously) that by coming between you, she can make a bigger place for herself in his life. Unfortunately, that behaviour tends to backfire.
I don't need labels to define me. They're sticky and I hate the glue they leave behind.