There're a lot of meanings:
"It's time to rock-n-roll!"
= It's time to begin! Let's do this right now! Let's go! ...etc.
= Let's do our performance! / It's show time! / Let's start this big project! / Let's do this challenge! ...etc.
---Example: "Are you ready? Alright. Let's rock-n-roll!"
"Time for a little rock-n-roll!"
= It's time for a little fun / it's time to go have fun!
= It's time to relax / it's time to loosen up!
---Example: "We finally finished _____! Now it's time for a little rock-n-roll! Let's go to the _____!"
"Everyone has 'a little rock-n-roll' in them..."
= Everyone has potential/skills, even though it doesn't seem like it.
= Everyone has a crazy side / everyone can be crazy when it's the right time to act crazy at the dance party, concert, event, when you're home alone and no one can hear you sing like a rock-star (whether you can sing or not), etc.
= He may seem shy, but when he sings his voice is beautiful!
= It may not seem like they like rock-n-roll music, but when I took them to a rock-n-roll concert they loved it!
"...add a little rock-n-roll..."
= Be slightly different/crazy/energized (at the party, concert, sport game, etc.)
---Example: "Come on....add a little rock-n-roll! Don't just sit there."
Hope I helped you a bit. If you still have questions, then please let me know. :)