What are the chances of kid suffocating in a ball pit?

Almost None: Almost none unless choking on a ball.

Ball pits: Degree of risk is likely to be impacted by ball size. A 3.1 inch ball is optimum for children 3 & under. Small balls might be placed in mouth & become a choking hazard. Use of small sized balls or a mix of sizes can decrease the void space between balls thus reducing air space for a child submerged under balls. Did not see any reports of children who suffocated in ball pits when researching this.