V.B.
hi L.-
You already got some good responses, but as a music teacher (and mother of a toddler), I'd just agree to take the cues that your younger daughter is giving you. Unless there's some physical reason why she's in a structured gymnastics class, I'd say that it's better that you are planning to find something else for her to do during your older daughter's class.
When I take on new music students, I always meet the student before agreeing to take him/her on, especially if they are younger than 7. I've found that some students do well with structured instruction at a young age, and others don't - it's just dependent on their personality. I've met some parents who really want their children to start certain activities at a very young age, and while it works for some children, I'd say that more often than not, starting kids on activities before they are ready is more likely to turn them off the activity. And from personal experience, I know that most 18-20 month olds probably just want to explore the world at their own pace.
No judgment here - just my observations as a teacher and mother! Best of luck to you.