I learned on the ultimate adjustable-speed machine - a treadle Singer!
Perhaps start by letting her watch you - go somewhat deliberately (slowly) and talk about what you're doing and why. Give her (encourage her) to ask questions.
Do some sewing, then ask if she would like to try the seam you are on. Don't overcomplicate it early on with how to thread the machine, adjust tension, etc. Get the machine where it's sewing well then let her get the feel of it.
Mostly at that age, teach safety - keep hands/fingers away from the needle. Stop immediately if you're not sure about something of if the machine is giving your trouble - let her know it's OK to stop and check things out if it's not going smoothly.
Start with the fundamentals. Worry about precision later. If you put too many criteria up it can turn her off. You can tell her that quilting is usually so and so wide, BUT she is 5. That's a huge concept to grasp at that age. Easier to say "follow the line and try to keep it straight".
Find a small, simple project with straight lines that she can complete and have something to show. A square potholder. 2 pieces of fabric and some Insulbrite in the middle. Let her pick the fabric. Maybe do it as a 3-side stitched inside-out then turn and hand-sew the final bit. Then USE the potholder so she can see how you can sew useful things that are attractive, too.