Use staves to create a normal noughts and crosses board (4 staves in total).  Split the Six into two groups of three for each team (thus each Cub will get one turn to tie the knot). 

Assign each team the knot for their team.  You can use easier knots or a combination of knots.  Each team takes a turn to tie the appropriate knot and place it on the nougths and crosses board.  The first team to get all their knots in a row wins.  They can then swop sides (knots).

So - you can use bowline, reef and sheet bend for noughts.  Figure of eight, clove hitch (around a smaller stick) or round turn and two half hitches can be crosses. 

It doesn't matter how long each Cub takes to make the knot - they are going in turns so it is not a race :-)