If you use .resize(x,y,w,h) on the child and then .schedule(0,"scrolltobottom") on the scroll it should work fine
It just needs a tick to update the child height. There'll still be a tiny flash of the new height before it scrolls.
Also, look into GuiMessageVectorCtrls. You'd have to make a custom profile, set their font face and size on it, and set their font colors in .fontColor[0-9] to style it (and you can't do substyles), but when you push a line on a message vector it automatically scrolls the parent scrollctrl. Stupid hacked in functionality there, but it does work (RTB chats use these, last I checked)