I would think that a concrete structure of any size that's meant to be played or climbed on would need interior reinforcing. I've seen faux rock walls being made at zoos and parks . Basically they're a rebar & wire mesh form with concrete stucco applied over the surface, but they usually aren't meant to be climbed on.
Could you build the wall of cinder blocks set on a good foundation? The party guests could attach their decorations to that and you can follow up later with grout.
You might consider posting this question over on the hypertufa forum, lot's of good folks that know about concrete over there.
Good luck and let us know what you decide to do......and of course we expect pictures!
: )

If wall built it out of concrete blocks, good foundation you can use thinset to glue the tiles on, then grout, seal if you want. I and another teacher wrote a grant to do a 50'snake. He did the major building and I did the mosaic. He built it using rebar, chicken wire mesh ( various layers and concrete ). You may want to use another kind of wire with smaller openings. Create a design, pick location, place rebar, start sculpting the form with the wire, attach the pieces of wire mesh, with other strong gauge wire. After scultpure is complete start covering with concrete mix. make sure to smooth out the area before it dries out.1 section at a time.Do concrete in one day. I was planning on doing a bench shaped like a turtle. Then mosaic. Good Luck!