Kind of late to answer this, but I thought I'd give you my two cents since no one has replied...
We have a "fit disc" that I bought on Amazon. The ones they sell on the special needs websites are basically identical but more costly. Ours has ribs on one side and nubs on the other, so my kiddo can chose whether he wants smooth or bumpy. We have one on his seat in our kitchen and he uses one at school too. He has moderately low tone in his core and can be kinda floppy when he's sitting. The disc has helped his slouch a LOT and also improves his concentration. The only caution I have is that, if you use the disc on a normal-height chair (as opposed to a child-sized chair), you'll want to make sure your daughter has something she can put under her feet. The disc will boost her up a few inches and, unless she's VERY tall, her feet will probably dangle in the air. This makes sitting on the disc really tricky and unstable. I'm short and tried using the disc myself and couldn't because it was so wobbly. My son has a Tripp Trapp chair, which has a built-in platform for his feet. If you don't have something similar, you'll probably need a footstool.
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