Set a time, or set a number...or even better let your student choose. For example a conversation may go something like this...
Parent, "Which section or part of this piece is the hardest for you?"
Hopefully student will give a good reply although the silent treatment is common at our house.
Parent, "Well, let's put a little extra attention into that spot so it isn't. Would you like to practice it for 5 minutes or 10 times through, you get to choose".
Of course vary the time or number of reps depending on age and level. For my youngest daughter it's usually more like, 3 minutes or 3 times through.
Child chooses, you set a timer or give them a stopwatch (my kids LOVE timing themselves). Or give them some little toys to use as counters if they want to count it out. Army figurines work great for my son, my daughter prefers pennies or pom pom balls. You step out of the room and let them go to work.
Extra praise and compliments when they are done with their time/counting is ALWAYS a great idea, but leave the correction and criticism out. I realize that there are times when you need to show them the correct way, or help them get it right. But give them some tasks every day when they are in charge and practice it their own way. Positive reinforcement in the form of praise when they have finished will go a long way in helping them do it even better next time. I have a friend who has many musician kiddo's, and she is so positive and kind in her remarks to them. It has paid off, her kids are all very talented and passionate musicians...and very well behaved to boot! My kids will be the first to tell you that I need to work on developing her same patience and positivity.