Custom model Entity's Can't have Baby Variant?

I've been making a new entity that looks like mooshroom but with different type of mushroom on top of it, I didn't find mooshroom model in the list so I made a new one!, Everything is going well untill I realised that I had to give it a baby variant but after my test the baby variant of this mob have same model as adult but it have smaller hitbox and all baby properties. I suggest making a new way to give custom model entity a baby variant either by making a new model or the ability to change render scales  or even giving the ability to give any mob variants instead of making another mob for every variant... This will help too much and thx.