You use a modelling app, such as 3DS Max, Maya, Lightwave, Blender, Wings, etc. They range in price from very expensive down to free. Steep learning curve and you'll find that making the model is only half the story. After that you have to rig it (so it moves) and texture it (so it looks like the thing you have in mind, rather than a plastic or clay model). IMO, if you want the big eyed child look, you'll find it easier initially to get the DAZ figures and morph packs, rather than use magnets on the included Poser content. As a shortcut, Thorne and Sarsa make many wide eyed child type characters for (mainly) DAZ figures.

Coppula eam se non posit acceptera jocularum.

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