?Pape mache? takes a while to produce & does not  bode well to touring & handling  - unless you have loads of tin lids (Kids) or students to do IT as an excise?
The quickest & easiest way using chook wire! Is  cover it in canvas or cheaper still unbleached calico (muslin). Add ?size? (wallpaper paste) if your are using a water based poster paint or pva wood glue in plastic paint. This will strengthen & retain the shape you have moulded the materiel into.  However mount it on a an mdf or ply board, so as to allow the materiel to be affixed & hold it?s original shape. As you said they are climbing over it, ensure an adequate wood support structure is under the wire & attached to the board.
Also you can use blocks of shaped Styrofoam shaped & covered & mounted the same way. The beauty of this is, it requires minimal building skills & will support being climbed over.
With the foam you may have to seal the surface or fix it to the material to stop it crumbling way - you can use the sized paint to achieve this, instead of buying a special adhesive.
 In both methods you shape the rock virtually as you apply the sized paint.
By using canvas type material - it can be repaired on the road by using a piece of ordinary roller bandage from the first aid kit & painted on as a touch up - rather than a messy Gaffer tape repair - that will always be seen for what it is!
Hope this helps you Andrea?

I am late to the game here, but having recently produced four papier mache trees, a boulder, and a stump I feel somewhat qualified :)

We started our project using traditional papier mache techniques, but switched to using fabric as a base quite quickly.  We were sort of making it up as we went along.

We found that creating a wooden armature of the basic shape (applied to a plywood base) covered with chicken wire.  The Chickenwire is fairly easy to staple to the wood, but wear gloves..  I then soaked muslin (only a dollar a yard) in a thinned white glue and molded over the top.  I used old sheets for some of the trees as well.

You want to be sure to leave a generous amount of fabric in the hollow places of your boulder. The fabric can be stapled down as well. The fabric will shrink somewhat as it dries.  It is a messy job, but the result is quite nice.  As to painting.  Use real boulders as a guide... you usually will need at least three colors if not four for good depth.  Greys, greens, mustards.... most boulders are not brown and neither are trees