For children to enjoy rhythmic gymnastics (and if they don’t, they won’t achieve anything) it’s worth you discussing the question with them yourselves. Perhaps your daughter is more interested in a different sport, or in dancing, figure skating or aerobics. You can go to competitions and concerts together, or else watch recordings of performances by figure skaters, gymnasts and different dance styles. It’s easy to find videos like this on Internet.

As far as “it’s painful” is concerned, I don’t think there’s any point in discussing this in advance and frightening the child. Let her first choose what she likes best; let her start practicing and enjoying it, getting pleasure from her new activity.

When it’s time for the stretching – and this is essential both in sport and in dance – you have to explain to her that any real result or real achievement requires hard work and discipline. Yes, there are some complicated movements that require special training, special exercises, and if you aren’t used to them, the muscles and ligaments can be sore. Pain is a barrier that you need to overcome. If the child has a low pain threshold you can talk to her trainer (teacher) and start doing some exercises in advance to prepare her muscles for stretching so it’s less painful for her.