Have never done it- not to my taste- but observe that it seems always to be 'flat' work, i.e. open voids filled and flattened. Others mentioned small, almost flat objects embedded. I would be concerned about bubbles with something as 3D and intricate as toy tractors. Would definitely do some experiments first.

The project linked by jwal10 was a diorama with a sheet of clear glass over it. Not what the OP is talking about.