Based somewhere after the part where Manny lets Peaches and Louis go investigate the island at the end of Ice Age 4:

...

"Whoa! Look at that!" Peaches pointed at a few cave paintings on the side of a cave. Louis took a step back. "Doesn't look safe, Peaches," he said cautiously, "The only creatures that paint like that are humans. Maybe we should get outta-"

"Hey! What's that?"

Peaches nodded her head to a cave painting that depicted two men playing a game, and strange beasts surrounding them, like sea serpents and ghosts of unimaginable shapes. The artist had captured such horror on the men's faces, even Peaches and Louis were frightened.

"We should really get out of here, P-Peaches." Louis whined.

Peaches agreed, but just as they turned to leave, a strange drum beat filled the air.

Louis panicked. "THE HUMANS HAVE SPOTTED US! RUN!"

"Wait."

Louis looked at Peaches like 'WHAT?!' but said nothing.

Slowly, the teenaged mammoth and her mole-hog friend inched into the cave. Suddenly, the drum beats flared up.

The two teens jumped, each letting out a shriek. Out of the dirt, almost completely buried, lay a box, and strangely enough, the crazy drum beats were coming from the box.

"Come on, Louis!"

"Uh...P-P-Peaches-s-s..."

Peaches uncovered the box and, suddenly, the drum beats stopped all together. The mammoth teen cocked her head.

"Cool!...Uh, what is it?"

"I don't know."

"Well, I'm opening it."

"Peaches! No!"

Too late. The box was open.

Louis cowered behind Peaches's front leg, shaking, until he heard her laugh.

"Silly! It's just a board game!"

Louis looked, and sure enough, it was a board game. In the side panels, four little play pieces lay, chipped and seemingly very old. In the center of the board was, what looked like, a little glass portal that looked like a solid green pond. On the side panel was instructions. Peaches read them aloud.

"Jumanji. A game for those who seek to find a way to leave their world behind."

"Yeah...rrrrrright. Heh heh. Well, that's very interesting and all, but I'm not playin'."

"Awww. Come on Louis! It'll be fun!"

"No. It'll be trouble. I'm not playing."

"Awww, Louis!"

Peaches had picked up two game pieces, but now she tossed them down and followed her mole-hog friend as he left the cave.

Behind her, she heard a clicking noise.

Peaches turned around and, to her shock, found that the game pieces had righted themselves in their starting places.

Louis noticed this also.

"O-okay, that's weird..."

"Please Louis? Just one round?"

"*sigh* Okay. Fine. But only one."

The two of them sat down on either side of the board. "Wait," said Peaches, "There are two more pieces. We should get two more players."

"Well, then you're gonna need to get two somebodies who love to play. Two somebodies who are stupid enough to play a game like this."