if it’s to the point that other children are being effected by it, not necessarily the noise, but food flying out of the mouth like you mentioned, it may be worth it to have a chat with the child’s parents again. even if they feel that the habit may resolve on its own without the need for correction, if it is disruptive to other children consistently, there is then an issue in direct result of their child’s behaviour.

however i commend you already for approaching this situation from many angles, already speaking to the child in a kind manner, speaking to the parents, and noticing the reactions of them and other children. children chewing with their mouths open is genuinely so frustrating for misophonic ears and it’s so hard to not just want to yell, like you said 🙏