how to build a cave of prehistoric times

Not sure what to do about the figures, but if I was building it, I'd build up the cave mouth with modeling clay and then coat the cosmetic surface with either Celluclay or epoxy putty. Since your kid is involved, you may want to use the Celluclay, since epoxy is kind of dangerous if not used properly, plus the risk of skin irritation. The Celluclay will also be less work. It will have a nice stoney texture when cured, whereas you'd have to apply a texture with a rock or some sand paper or something with the epoxy.

 If you did it in 54mm, and could find the play set, there's the old Marx Prehistoric playset someone mentioned in a similar topic a few weeks ago. Might be a good source for figures.

For Dinosaurs your best bet will be the Tamiya range of kits in 1/35. But than few lived with pre-historic humans, but they do include alligators, etc.

I would do the basic cave layout in styrofoam covered/extured either with plaster of paris and/or texturing gels(both are safe for kids).

Pre-historic humans might be tougher, but you might find one or two wargaming/fantasy pieces that might fit. Or you might be able to mod some 1/35 scale figs.
FYI, 54mm is about 1/32 scale.

Alternatively you could always do an empty cave with a bit of "wall-art", cold fire-place and everybody out for the hunt/scavenging.

In one of the museums I visited they had a nice 1/1 display of a pre-historic hut made from Mammoth teeth and skins/earth to fill the gaps.

HTH.

Skip the dinos completely since they were extinct 64 million years before the earliest proto-humans were around. You'd be looking at early horses, mammoths or mastodons, bison, cave bears, dire wolves, sabre-toothed cats, giant sloths, wolly rhinos, etc. Playsets are about your only option. Try www.hobbybunker.com or www.michtoy.com . Both have good selections of 54mm recasts and you could probably get one with some animals as well.

i would first head to a toy store and see what you can find there.

thank you,will do

some 54mm pewter wolves can be used as those tamed ones which... later on now we call them dogs~  cool dio idea, "Men's Best Friend" <-corny

Also, Model Railroad shops have some kind "kits" that include all the materials to make mountains and rocks, looks like a small step from a rock to a cave.

Indeed, dino's were gone waaaaaaay before we arrived, sabertooth, mammuth and bears were our ancister's main enemies/preys.
About the appearance of the cavemen, well, you can go two ways actually.
The traditional way (19th century) is that they were big, hairy and ugly,
however, cerent studies of their skulls and skeletons have reveiled that even Neanthertall were not much different in appearance than modern Homo Sapiens, in fact, some paleontologists suggest that if you were to put a Neanthertal in a small group of Homo Sapiens (not Homo Hollywoodiensis or Homo Modelliensis Cameratus ) that they really wouldn't stand out all too much.