I'm working on a 'Tortoise' mob, and it's almost finished. It can breed and have babies, and though the babies' hitboxes are small, their models render the same size as if they were adult. What do I have to add to my entity/model/render classes to make babies render smaller than adults?

If you look in the vanilla render classes, you will see that they set the scale for child versions, I think during the pre-render callback method:

GL11.scalef(0.5F, 0.5F, 0.5F);

 

EDIT: Actually, vanilla mobs do it inside of the model class (see ModelQuadruped#render, where model#isChild is set by the render class), but you can do the same thing inside preRenderCallback if you extend RenderLiving.