Student or Learner
Whenever I dress my child up, he moves a lot and he doesn't stand up straight. I usually say:
"Stay still" or "Stand straight". Are my sentences correct or should I say something different?
Stand up straight
Note: Simply putting clothing on your child is "dressing your child." When you put nice/formal/good/fancy clothing on him, you are dressing him up.
This applies to dressing yourself as well. "We had a lovely dinner at [name of fancy restaurant]. It was nice to get dressed up and have a grown-up evening without the kids for a change."
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.