How do you say "ball of foot" in Chinese?
Difficult!

How about 脚球？（开玩笑）

The bones you are talking about are the 跖骨

https://baike.baidu.com/item/%E8%B7%96%E9%AA%A8/4055417?fr=aladdin

Can't really find a word for 'the ball of your foot', seems it is not a Chinese thing. The second joint of your thumb has no real common name in English either.