The technique I've used to fill a balloon with rice:

- Take a small plastic drinks bottle.

- Cut the bottom half off to make it like a funnel.

- Remove the lid, and stretch a balloon over the neck.

- Invert the half bottle so the balloon over the bottle-neck is hanging underneath.

- When you tip some rice into the funnel, only a few grains will fall into the uninflated balloon.

- Now put your mouth/cheeks against the open end of the bottle/funnel and blow. This will partially inflate the balloon, and all the rice will fall in. Done!