I can maybe help you.>-how to draw this dumb mouth at this dumb angle
You could cheat a little and draw the interior of the opposite cheek like pic related.>-how to give kids expression without adding wrinkles and making them look old
It's hard, study kids faces and narrow it down to the essential lines :
Kid is angry ? Frowned eyebrow, eyes wide open, maybe mouth closed and crisped.
Kid is afraid ? High eyebrows, eyes wide open, mouth open.
Kid is being happy ? High eyebrows, eyes closed, smiling.
No need to put additional lines. I don't see why you would.>-how to draw dumb messy hair without it looking retarded
I'm not good enough to do it myself but you have to study how the hair goes from the scalp and draw accordingly. Maybe this video can set you on the right direction? https://www.youtube.com/watch?v=7L9VJSr_PEI
. Good luck with that.>-why do i destroy half of all my sketches when i try to simplify it into black line art
Because black line art is hard as fuck because of line weight, silences etc and you need to practice it and experiment a lot to get it right