I tried out some of your suggestions this week, and I thought I'd let you know how it turned out. But first of all, a warm thank you to everyone who contributed thoughts and comments. I really appreciated them. :)
This week's lesson was about blessings. There were word strips (with things such as "The Gift of the Holy Ghost") on them, and a story, etc. Each word strip, I wrote out the English and the Spanish underneath, and when we would get to putting up the word strip I would say both the English and the Spanish (happily, Spanish is close enough to Italian that my pronunciation is fairly good, even when I don't know exactly what I'm saying...so when I came prepared, I was fine!).
In addition, we learned the 4th Article of Faith in both English and Spanish. I would have the kids recite a small portion of it in English, and then redo the same portion in Spanish. It was fun, and got her and everyone else involved in speaking her native language! I thought that was a really fun activity.
I also went with your guys' suggestion to make sure to use plenty of pictures. I used a lot of pictures in my lesson (we talked about Enos, baptism, confirmation, etc) and I learned enough "keywords" in Spanish before Sunday that we could talk about those subjects.
The Church has so many resources!
Again, thanks for all your suggestions, and I'm sure I'll continue to look at this thread whenever I run into problems or stagnation. But already today, she was more talkative than she'd ever been before and seemed to follow along much better with the lesson. One of the parents of the kids in my class also speaks Spanish, and since his child frequently refuses to be in Primary without his dad, his dad is in my class many Sundays as well. Happy coincidence (or...blessing in disguise :) )