Deciding on a suitable theme might be result of a group discussion but the end result will need to be a style of image that is easily read in outline, with only a small amount of detail needing to be drawn in on the front surface. These images were collected in four minutes using the search strings "fish cartoon", "shark cartoon" "fisherman cartoon" and "fish sketch". They could be the starting point for a personalised sketch in the same style. It would also be worth giving a little thought to the backwards and forwards rocking motion that these toys have when moving. Some images will be better than others when that rocking is applied to them.
Remember it is only the outline that is important at this stage.
Balancing the shape you have drawn has to start with understanding how simple levers work. You started understanding this when you first went to a playground in which there was a see-saw - even though you probably didn't know you were doing it !
If you think of the see-saw as a simple sum it is possible to calculate how heavy something has to be to balance a weight already on the see-saw. If the big person weighs 2 units of weight and the little person weighs 1 unit of weight then the sum is easy.
2 units x 1 metre = 1 unit x ? metres
Try using this calculator to play with different weights and different distances - Until you are confident you understand what is happening when a Class 1 Lever ( like a see-saw) is balancing.