The kids at our gym don't start actual tumbling until they're 7 or have been in gymnastics for several years and they're ready. Tumbling is much harder on the joints and body than just regular gymnastics.
I think if she's invited you should try it and see how it goes. My kiddo has been on the gym team since she was about half way through her first year in tumbling. She started gymnastics when she was 3.
There is a lot of class time before they start competing. I'd look at this a different way than others though.
Google "the benefits of gymnastics for kids" or "The benefits of tumbling for kids". I was amazed at all that it does for the growth of the bones and muscles.
They do spend all the class time on the Tumble Trak and the springboard floor. She misses the uneven parallel bars so we let her take an additional gymnastics class too.
One of our kids take 5 hours per week and the other takes 2 hours per week. They do dance and gym classes. They love it and I expect they'll stay with it through school and perhaps get scholarships to college. Our facility doesn't have anyone that graduates who needs a scholarship and doesn't get one based on dance or tumbling. They are THAT good. One of our most recent graduates made the national college cheer squad her first try out. As a freshman. Another was a guy and he got a full scholarship on tumbling and was n their cheer squad. He's a fireman now and still comes back and does exhibitions with the team for fun.
As for the dancers, the most recent graduates have gone to Julliard and she received an awesome scholarship, another went to a state college and she's already working with that state's premier ballet company.
Kids who love the sport of dance or tumbling or gymnastics almost always do something with their skills as an older child, teen, and into college. I think finding a good balance and see where it goes is a good first step.