This was one of the simplest and cheapest accessories I created for the space. For the trunks I used a couple of dollar store pool noodles. Then wrapped the trunks with a fun pattern duct tape. There's so much crazy tape now in craft stores and hardware stores, it added a little extra whimsy than plain solid tape would have.
Once my trunks were taped, I made the tissue paper tree tops using this Martha Stewart tutorial. I'm going to level with you, I hate making those tissue paper pom poms! It's simple, but frustrates me. I know some craftier people crank them out like it's nothing, that's never been the case for me. There's just something about peeling apart the tissue to fluff it up that I can't handle! I'll rip it every time. Luckily, you can't really tell once it's fluffed, so just have it. It was worth it in the end. Another thing you could try is to buy the pom pom kit at a craft store or party supply store. They're not that expensive and may save some hassle.
To secure them to the wall I just bought a package of Velcro strips, placed a couple on the noodle and the wall and just slapped it up. Velcro tape is definitely under utilized in design. Every once in a while one will fall off, but I stick it back and it's fine.
The thing to remember is it's not art. It's $2 of pool noodles, $4 of tape, tissue paper I had on hand and probably $5 of Velcro tape. If they get torn up because kids are playing like kids, we can rebuild.