Personally, I don't think the chair the baby is sitting in should be too hard. There is a kind of chair with a little sponge on it, which is neither too hard nor too soft.

Make another small advertisement > & Learn more. The learning machine for early education is specially designed for babies aged 0-8. Learn to tell stories, learn to paint, learn English, learn to sing, super multi-functional, no radiation. I suggest you try.