Try out three basic animation techniques: Stop Motion, Flip Book, Thaumatrope
Digital camera, tripod, lego/playmobil characters, post it notes, pens, cardboard, paper, scissors, glue, string
Could just do one of these activities but to keep Beavers busy it's good to do all three and rotate lodges around them so they get a go at each.
Introduce the idea by getting them to wave their hands quickly in front of their faces so they can see the blur caused by 'persistence of vision'. The brain holds on to images after they've actually moved, that's why animations and movies work.
Use post it notes and pens to make short animations on the edge of the pads. Encourage them to think of action moments like bat and ball or incredible shrinking man.
Make a Thaumatrope. http://en.wikipedia.org/wiki/Thaumatrope
This is a disc with a string threaded through it so it can be spun rapidly. There's a different picture on each side and when it's spun quickly it merges in to one image. Popular combinations are bird and cage or cat and whiskers.
Cut disc out of carboard, stick paper drawings or download examples from the internet, e.g. http://www.minieco.co.uk/diy-thaumatropes/
Stick them on and use hole punch to make holes to put string through. Simplest to use a single length of string and spin between finger and thumb.
Take care to put the drawings the right way up on each side of the disc or they won't line up when you spin them.
Stop Motion. Set up camera pointing at a 'set' and get Beavers to take a picture and make small movements of their characters each time. Most cameras allow you to play back stills fairly quickly by holding the forward button down, this will show your 'movie'. There are plenty of apps that will automate for you as well.