Tandem Cycling - Input on Child Stokers and Cadence

I am seeking input on child stokers and cadence. Recently when my 4 yr old daughter was on the back of our tandem, someone exclaimed, "look at her legs go." This raised a concern that the cadence was too high. I am interested in others experiences and whether and what type of injury risk there is.

My daughters inseam is 22".
The stokid cranks are 120 mm and are in-sync (i.e. 1:1).
She has swept back handlebars that provide an upright position.
My typical cadence is 100+ RPM's, but ride the tandem between 80-90 RPM's.

My grandson is 5 and his cadence is 75-85 range with short burst's up to 100. He's strapped in but I'm always asking him questions so I know he's alright with the cadence were at. On really long stretches I'll go down to 70 to give him a rest for a bit.

I have observed that kids can spin like crazy if their cranks are anywhere near an appropriate length. We started our (petite) daughter on a triple when she was 3.5-years-old. Her cranks were 110 mm. There was no need to drop from our habitual 100+ cadence. Now she's eight and I've moved her pedals to the next (140 mm) pedal position on her cranks. That may be a touch longer than optimum, but she only reports bouncing when cadence gets to approximately 120. As long as your daughter feels free to speak up when she's uncomfortable, you should be fine.

Enjoy!

+1 on the comments above. I recall reading on the performance tandems website (which btw has lots of good info on how they rode with their kids as they grew) that small kids are much happier spinning than pushing.