Homework can be as monumental a task for parents as it is for children. So what's the best strategy to get a kid to finish it all? Where's the line between helping with an assignment and doing the assignment? And should a parent nag a procrastinating preteen to focus -- or let the child fall behind and learn a hard lesson?

As schools pile on more homework as early as preschool, many parents are confused about how to assist, says a 2012 research review at Johns Hopkins University. Some 87% of parents have a positive view of helping with homework, and see it as a beneficial way to spend time with their kids, says the study, co-authored by Joyce Epstein, a research professor of sociology and education.

Yet sometimes parental intervention may actually hurt student performance. During the middle-school years, such help was linked to lower academic achievement in a 2009 review of 50 studies by researchers at Duke University. Parents who apply too much pressure, explain material in different ways than teachers or intervene without being asked may undermine these students' growing desire for independence, according to the study, published in Developmental Psychology.

A parent who implies that a child isn't capable of working on his or her own 'causes the kid to lose confidence, to get mad and just want the whole experience to be over,' says Lisa Jacobson, founder and chief executive of Inspirica, a New York tutoring and test-prep company. When parents help too much, 'kids say that they feel like a fraud,' undeserving of the grades they receive.

Kids also need different kinds of help at different stages. In elementary school, parental rule-making about where and when homework is to be done, along with encouragement, is linked to higher achievement. But parents should give advice or help only when asked, says Harris Cooper, a professor of psychology and neuroscience at Duke University and a leading author and researcher on homework. If a child fails or becomes frustrated, parents should suggest a break.

Some of the best ways for parents to help, Dr. Cooper says, include providing a quiet study place, proper supplies and resources for doing homework, and instilling positive attitudes about learning in general. A good motivator is to show kids how the skills they are practicing might be used throughout life.

Shawna Mazeitis tries to keep learning fun for her own children, aged 15, 14, 12 and 6. When her 6-year-old daughter Paige struggled with a computer assignment on fractions, she burst into tears 'and didn't want to do it anymore,' says Ms. Mazeitis, of Parkville, Mo. She encouraged Paige, saying, 'I can see you're upset. Sometimes when we're upset we can't think clearly. I know you can do this. We'll come back to it a little later.'

Later, Ms. Mazeitis took out a wooden pizza puzzle to refresh Paige's memory on fractions without having to type on a keyboard. Paige soon returned to the computer and finished the assignment. To keep things fun, Ms. Mazeitis also switches roles with Paige, sitting in a small chair and playing the student, while Paige stands by a chalkboard, pretending to be her teacher. By middle school, when studies show students crave more autonomy, parents can coach students on problem-solving strategies. When Lorraine Landon's 13-year-old twins Cienna and Keenan became frustrated with middle-school math homework, she told them, 'I'm not going to rescue you,' says Ms. Landon, of Pasco, Wash. Instead, she urged them to review what the teacher had said in class, consult their textbook, or make an appointment to get help from their teacher

Once high school rolls around, parental involvement is linked again to better academic performance, according to the 2008 Duke research review. That may be because parents at this stage tend to get involved only in subject areas they enjoy -- passing on their enthusiasm. A 2011 study of 12 high-school classes published in the School Community Journal found students paid more attention in science class and spent more time on homework when their parents were engaged.

If parents dislike a subject, they can easily pass on that attitude too, says Bon Crowder, a Houston teacher, tutor and publisher of MathFour.com, a website on math-teaching strategies. A student may think, 'Omigosh, my mom who is 40 is freaking out about this, and I'm 14 and supposed to be doing OK with it? How can that be right? Maybe I should freak out too,' Ms. Crowder says.

A better approach, researchers say, is to guide a student toward finding help elsewhere, such as video tutorial websites such as KhanAcademy.org, or to ask a tutor for help.

Some homework problems spring from the assignments themselves, which can be confusing. In three studies in 2009 and 2011 by Frances Van Voorhis, a research consultant at Johns Hopkins' Center on School, Family and Community Partnerships, students who did homework structured to involve parents in a positive way said the assignments eased stress. The students posted higher test scores compared with control groups.

Susan and Daniel Buckles, of Parkville, Mo., use homework to instill long-term study habits. When Ms. Buckles realized recently that her 11-year-old daughter Annie had been procrastinating for weeks on a project, she stepped in -- but only to help Annie set up a plan, breaking the remaining work into manageable pieces.

Annie is grateful for the 'wake-up call.'

Her parents, she says, have taught her that 'if you don't nurture and grow your study habits to be the best that you can be, you're going to fail college, you're not going to get a good job and you're going to be sad, thinking, 'Why didn't I study more when I was younger?'

Sue Shellenbarger