Cello is a demanding instrument, requiring correct ergonomics to avoid pain and injury. It's important to select a chair of the proper height for each player.
When sitting towards the edge of the chair, separate your legs like you have a cello between them, and put your feet flat on the floor. The tops of your thighs should be sloping down slightly from hip to knee. That will be the correct height for you. But everyone is different; you can't put your 8 yr old child on the kitchen chair and expect that to be a good fit. And littler kids are particularly hard to find a suitable chair for.
We really like the AdjustRite chairs and stools. They are light-weight, and easily adjustable for different sizes. The stool will fit most 5 yr olds, and the tall chair will accommodate people over 6'. The chairs also fold up for easy carrying or storage. The seat is designed with a slight slope forward for proper back and hip placement. If you want to use the chair for other things, you can flatten the seat by simply lowering the back legs one notch.