My daughter skipped kindergarten at a school with a composite 1/2 class. She was aware of what was going on from the beginning, because they tested her to make a decision about the skip. She was very enthusiastic about the idea. For her, it was a big adventure.
You could ask your DD what she thinks about the idea generally without mentioning that you're thinking about it for her. When we brought up the subject with our DD, we told her about the level of work in kindergarten (e.g. learning to identify numbers and letters; she was doing addition of two-digit numbers in her head and reading chapter books when they tested her the spring before she started). This information helped her make an informed decision (e.g. "I do NOT want to learn about the letter B!").
