But wouldn't it be better if your mother and girlfriend spent more time together so she wouldn't make those mistakes? Where she could be shown things like how you like your sandwiches cut or how many sugars you like in your coffee because let's be honest you probably don't even know if you take sugar.
Why not suggest it to her? And If she's upset, simply tell her you'll buy her some crayons if she continues to act like a child. She'll immediately reflect on her behaviour and calm down.