In the 1948 film "Sitting Pretty," the snide, know-it-all babysitter Belvedere (Clifton Webb) quickly wins over the three previously rambunctious children of Harry (Robert Young) and Tacey (Maureen O'Hara) King.

These days, not many kids would be called rambunctious — and that's too bad.

Instead, most kids are just sitting around, staring at their smartphones or computers. One study found that 8- to 18-year-olds spend an average of seven hours a day looking at one kind of screen or another.

And according to researchers, that can lead to big-time depression and long-term behavioral health issues.

Another recent study from University College London and published in Lancet Psychiatry found that inactive, screen-bound kids had a higher chance of developing depressive symptoms at age 18 and beyond, compared with kids who were more active.

The researchers say just 150 minutes of activity weekly makes a difference in kids' moods.

We also know that sedentary behavior early on can lead to physical impairments (obesity, heart disease, etc.), but this study underlines the mental health repercussions of not getting enough exercise when you're young.

To boost both your child's mental and physical health, help him or her get a mixture of moderate and vigorous exercise for an hour a day, seven days a week.

How can you do this? A study in the journal Human Movement found that sports activities — whether it's on a team or individual, or played indoors or outside — protect against depression.