Making a stress ball is really easy. They sell in the store for about $2.99 but you can make them for cents. The tools you'll need are simple but just like the stress balls at stores if you have a kid that chews on things this probably wouldn't be the best idea for them.
a filler (rice, sand, water if you don't mind if they break, gel, cornstarch, flour, small smooth pebbles, ect)
Funnel (not needed for all fillers)
Take the balloon and begin filling the balloon with your choice of filler. Keep pushing filler in even when it reaches the top until you get the desired firmness.
Take the balloon and tie it in a knot. Cut off remaining top.
Take a second balloon to wrap over the filled balloon for added strength. You may tie it or you can use several balloons with small cut holes of different colors for a more colorful and unique stress ball.