The yellow bellies are based on
Beipiaosaurus, a small therizinosaur from early Cretaceous China. (It says so on the back of the DVD case.) Of course, the real
Beipiaosaurus was covered in primitive, hair-like feathers (not just tufts on the head and tail), and had three (not four)-fingered hands with long, scythe-like claws on them (in spite of this, it was a herbivore). I suppose the LBT people thought the claws would look too scary. <_<
The tinysauruses were probably based on
Mussaurus ("mouse reptile"), a prosauropod (an early relative of longnecks) that is so far known only from fossils of hatchlings and juveniles. Adult
Mussaurus were almost certainly much larger than Big Daddy,

but I am more than willing to attribute this decision by the LBT filmmakers to artistic license.
For the record, I rather enjoyed LBT XI,

though I think it could have been more interesting (or at least made more sense in terms of the dinosaurs' initial disgust towards the tinysauruses) if an altogether new kind of dinosaur was used, rather than simply a miniature version of a familiar type.