One year I was a spider for Halloween. I used kids black tights for the extra arms. My arms and legs were 4 of the 8. So I only needed to make 4 extra limbs. I stuffed the tights with polyfil and used fishing line to attach 1 set of arms to the sleeve of my real arm so when I moved so did 1 set of extra arms. It was not too heavy.
Ursala has regular arms and her mermaid tail was an octopus instead of a fish. So maybe you could just attach two of her tentacles to your daughters arms and let the others just bob about and move with your DD movements on stage. The wire is a good idea. I did a search and got a great hit. The maker used clothes hangers. Here is the link....
http://www.coolest-homemade-costumes...a-costume.html