Asking for Help: Nap for Two

Hello! I need help, and after exhausting my own mind, the Internet, and a few other people, I'm turning to you.

Here's the dilemma ...

I don't know how to get my 4 month-old daughter upstairs to her crib to nap with my 22 month-0ld son around. When it was just the two of us, I could get him down no problem (and check on him as needed) because I had no one else in tow. But now he's in tow and she ends up sleeping on the main floor, behind a chair (for a little shade), in her bouncy seat; I leave all the lights off for most of the day, rain or shine, so she will have as close to darkness around her as I can get it.
Baylor won't walk up the stairs with me very easily, and I don't like focusing on her all the while worrying that he's going to fall down the stairs. When I leave him downstairs, he cries a bit and then tries to go upstairs on his own. He's tall for his age and can climb over and out of almost anything, and while he hasn't figured out how to climb out of his crib yet, he only tries when he's really pissed off and I don't want to give him cause to climb.

So how do you do it? How do you get the youngest kid down with the older one around (and awake)? Ultimately I'd like her to nap in her room in her crib; it's dark and quiet, and it's where she should be sleeping. But it's on the second floor, and the stairs are unguarded at the bottom.

Any help or ideas you could give me would be wonderful! Thank you so much!