How can I explain this to a little kid?

[Volleygirl22]

I'm over at my boyfriend's house today. It's just me, him, and his youngest sister. (She's 5, and adopted). We're all kind of stuck wherever we were, since there's a huge snowstorm coming through, so it will just be the three of us for tonight also. His sister asked me if she could sleep in my boyfriend's room with us. It sounds like she usually asks that when we're expecting thunderstorms and stuff like that. But earlier today (the reason why I came over) was because I got a text from my boyfriend saying "maybe we can make up for lost time if you want to ". Yes, I want to. But I can't really use that as the reason to tell his sister she can't sleep in the same room. How can I explain it to her without saying it's because we're planning on having sex, and without hurting her feelings? She's really sweet, and I don't want to say anything to make her feel bad. It's just that it's been a while since my boyfriend and I have had sex, and since he says he's finally feeling better and wanting to do it, I REALLY REALLY REALLY don't want to let this chance go.

[Sunshine]

I see two options here: either you tell her that you and your boyfriend really need some time alone and that's why she can't sleep with you. Or you let her in, wait until she's fallen asleep, then sneak out and have sex somewhere else in the empty house.

Having once been a little girl myself who was terrified of storms, I prefer the latter version. I am pretty sure my parents did this once or twice. Besides, if you try to make her go to bed by herself when she's afraid, chances are good that she'll be knocking on your door sooner or later anyway.