My theory is that Noah prefered to take the 2 foot beavers into his ark, instead of the 6 foot variety, they would take up less space and less food.
Although it is not necessary to believe that Noah had dinosaurs on the ark, the flood theory would certainly explain why all the dinosaurs died out in one generation.
Besides satisfying why the little dinosaurs did not survive with the big ones, you also have to account for everything else that did survive. Eg, crocidiles,turtles, and ants and cockaroaches, etc which everyone agrees were concurrent with the dinosaurs.
Again, the flood theory holds up. After the flood, only those reptiles who "hid" their eggs(by burying them ) e.g. crocidiles would survive, because the mammels let loose from the ark would quickly eat up any dinosaur eggs lying around on top of the ground- thus all dinosaurs which layed eggs on top of the ground died out in one generation - didnt matter if they were big dinosaurs or little ones.
The meteor theory which said that big dinosaurs could not find food, does not account for why the little dinosaurs which required very little food also died out. A world wide disaster which resulted in little vegetation does not explain why tiny dinosaurs survived.
As far as why we think the earth "appears" to be so old, is because you cant make a mountain in one day, and make it look like it is one day old.
One single minute after a mountain was made, it "looked" a billion years old, not a day. One second minute after Adam was made he looked like a 30 year old male, not a one second old male. There is no way to make a 30 year old male appear to be only one second old.