Are leg press and leg curl machines used to thin legs?
Yes you can, provided you use the correct exercise regimen with them.
Do Legs using these machines not more than twice a week, each session separated by at least 72 hours.
Stick to lower weights (with which you can manage the prescribed reps) and higher reps (around 15+ - Start with higher reps say 25, and do the last set with 15 reps, use only that much weight with which you can manage the target reps for that set)
Always do Leg Presses before Leg Curls so that you do not tire your quads before presses.
For leaning the legs, especially when you are female, you won't require much effort unlike men. Due to lower testosterone levels in females, you are naturally geared towards forming curves rather than growing muscles significantly.
If you have a lot of fat deposition on your legs, exercising on these machines will very marginally help you. You will then need to resort to doing cardio to lose the fat, because spot reduction doesn't work mostly.