I can see that some people have taken offense to you asking this question here, but I'm not personally offended and I will share my experience with speech therapy.
First of all, make sure you never, EVER discourage sign language, and always do your best to educate parents about why it is good to let their child learn/use sign language along with speech.
That said, as a child I HATED speech therapy. It was SO boring. The problem is no one takes it seriously enough when a child is bored. They just dismiss it. But it is a big problem, because speech therapy is something that goes on for a long time so after a while it is no better than outright torture for some kids. And that was the case with me.
Sure I speak almost as well as a hearing person and I am thankful that is the case but would I allow it to happen again if I could go back in time and change anything? No. It wasn't really worth it. I think if I needed/wanted to learn to speak so that hearing people could understand me, I could have just as easily sought out a speech therapist and learned as an adult. I feel it was unnecessary and perhaps even cruel to inflict speech therapy on me as a kid when I didn't want to do it. I mean, I was getting made fun of for the way I talked and I knew speech therapy would help with that, but I still didn't want to do it. That's how bad it was, and not just because it was boring, but also because it was a reminder that I wasn't "measuring up" to someone idea of what I "should have been" - the fact that one of those people was my mom made it much more painful.
Additionally my mother made it worse by INCESSANTLY correcting my speech. It made me not want to talk. But somehow, my being in speech therapy seemed to make her think it was OK for her to do that. Hint: it wasn't OK. So one more thing you should do is caution parents against doing this, tell them not to correct their child's speech and leave the speech therapy only to you.
And do your best to make speech therapy fun and then if the child still isn't enjoying it, explain to the parents that it needs to be something they want to do, because it is a lot more psychologically damaging to force speech therapy than most hearing parents think it is (and like I said, not just because it's boring, either - but because it's DEMORALIZING.) But at least if you make it fun, it may make the child WANT to do it and then it won't be demoralizing.
Hope that helps.