Here is a STEM kit that comes with everything you need to build a working engine with your kids. The Playz Flat 6 Model Engine Kit has over 320 pieces. Designed for kids ages 12 to 18, this kit teaches users a thing or two about engineering, including how horizontally opposed engines work.
This 6-cylinder engine makes real engine sounds. It ships with step by step instructions. You can find out more about it here.