I am certain I am not the only one who witness the sudden growth in the size of children sometimes, especially when they are doing activities such as stirring a pot or simply just leaning against a wall. I personally think this is a glitch (which is why I am posting it here) but it could also be an overlooked aspect in which those specific animations had only been optimized for advlt characters and thus children grow to their size in order to maintain likeness.
However I still find it creepy to see Frodnar the same height as my character