Du'a for tummy ache

لَا فِيهَا غَوۡلࣱ وَلَا هُمۡ عَنۡهَا يُنزَفُونَ

Laa feehaa ghawlun wa laa hum 'anhaa yunzafoon

Wherein there is no headache nor are they made mad thereby

Qur'an 37:47 (As-Saffat)