Plot hole: After the dinosaur kids wrecked the playground, Manny tells Sid, "Whatever they are, take them back" and someone says "What are they?" as well. Later, when Mummy Dinosaur appears, Ellie says, "I thought those guys were extinct!" and one minute later Manny says, "She's a dinosaur!" How could they possibly not see that the kids were dinosaurs if they already knew about dinosaurs in the first place?
Plot hole: The baby dinosaurs are small enough that Sid can carry them when they are still eggs, yet as soon as they are born they are twice the size they were inside the eggs. Their size shouldn't have changed that much, and even if it did the eggs would have been much too large for Sid to carry.
Plot hole: In the very end of the movie, when Scrat rides the rocket back to the ice world, it smashes a hole through the ice layer. Two mistakes follow: First, bits and pieces of ice fall away from the rocket. One can conclude that the hole is slightly bigger than the ice slab above, yet they fit perfectly together when the ice falls back down. Second, how can the ice separate from the rocket and fall exactly where it was before?