Good morning!

In the interest of full disclosure, I've never worked on The Miracle Worker. But, in general, with kids, whether they speak or not, I think it's more important to focus on what else they bring to the part.

If I were you, I'd have the kids do some improv games - nothing at all to do with the show, just stuff that makes them stretch their imaginations and encourages them to relax their inhibitions. Poke around on http://improvencyclopedia.org/ and find four or five games that sound like fun, but focus on physicality rather than verbal expression, since your actress won't have lines to carry her.

Then watch them. The one that's not afraid to put it all out there, the one that everybody else stops and watches...that's your Helen.

Good luck!