This really isn't a crossover so to speak, but the plot idea is a crossover from The Last Action Hero.
A young boy loves watching The Land Before Time every day. He even watches the reruns of the original in the old movie theater downtown. One day, the owner gives him a ticket that is said to have magic powers. The kid takes the ticket, but one day he is warped into the Land Before Time Universe (most likely after the first movie and in one of the sequels). The gang not only question what kind of dinosaur he is, but also how he knows so much about their world and their lives. Eventually, throughout the plot of the sequel, the kid returns to his own world, but he accidentally takes the gang with him. Now the young dinosaurs are in the streets of New York, scared out of their mind. How would they react to seeing no dinosaurs at all? What would happen if they learned that their world only exists in movies? Even scarier, the warp between worlds left a time gate open, connecting the two together, and one angry sharptooth walks through. What will the kids do to get rid of him? What happens when the time gate starts to slowly close, possibly leaving the gang trapped in the strange new world?