If you have a device and children, you need to turn restrictions on. How restricted you make your device will be up to you. Most people will want to at least restrict in app purchases, installing apps, and deleting apps.
Open your settings and find the "General" button. Click on that, then scroll down until you find Restrictions
Click "Enable Restrictions". This page can be confusing because the green means the app is "on" - the restriction is "off". I have turned restrictions on, or disabled three functions below:
Take a look at the other options there as well. Since this is my device I don't want to have everything age restricted! Keep in mind also that older kids can often find work arounds.
If you have a really little kid and want them to stay in one app or in one place inside one app, you will want to set up guided access so you can turn it on as needed.
Start out in the General section again, and find the "accessibility" line. Click on that.
Scroll down until you see "guided access"
You will be prompted for a pass code separate from your device lock (it can be the same if you want it to be). After that, anytime you want to keep the user on one app, triple click on the home button. Two choices will appear: voice over and guided access. Select guided access and then start. OR if there is a button in the app you don't want pushed (buy more games!) select options- you can draw a box around anything you want off limits. The boxes will not rotate with the screen though, so you will want to lock rotation, which is another option.