Is it bad for my 9 month old's posture when she's sitting on the floor with her head bowed down to play with toys for long periods of time?

For me it is okay to sit in such position for a short while (less than 30minutes). This posture will cause humpback in long term. You can get a kid sized table and chair for her to sit in a proper position. Ikea has a variety of kids table and chair (different designs and colors) which are very affordable. I bought Mammut Ikea for my kids and they love it!