If you're booking online you'll be able to check any age restrictions on events - just search the event and date you're interested in. Once selected, you'll see "Info" on the left, and the age restrictions will be listed there.
If an event does have an age restriction you'll need to abide by this. Even if an adult is attending, some events won't let anyone under 18 attend.
For events that do allow children, they must have an adult aged 18 years or over with them at all times.