Hello,
you are correct in stating the difference between creature and monster, but I think there is something else which needs to be included.
A creature is generally a created thing, so I suppose we could say 'real', and a monster can be a created thing, or pretend, as you've said. However, a monster is usually something which we find scary or terrifying. So, if the children aren't scared of the dinosaurs, then they won't call them monsters.
I hope this helps,
best wishes,
Annie