There is a young ancient in Emet's Amaurot recreation you can find if you wander around. My max height male Au Ra is significantly shorter than the shade, but he towers over Elidibus his last cutscene. That size difference would make him toddler sized, if that.
I don't think he was a child when he was sacrificed as much as him showing up as a child was symbolic for how one track/childlike his mindset was when you stripped away the thousands of years worth of hurting.

adults are more than twice as tall as my character

vs. child

Assuming he's using the same model as Unukalhai/other children, that'd make him around 1/2 my characters height