I have issues with therapists coming to daycare depending on how and what they do and what the intended outcome is. The ones that come and work with the child but also include the other children so it just becomes a normal part of play work the best. Issues I have had with some is they needed to work one on one with the child so it either meant I had to take the other kids to the basement playroom and leave them upstairs which was my preferred or the therapist would ask to go downstairs so they could use a particular thing and while I let them it annoyed me.
What worked the best was to have the parent take time off and keep the child home on therapy days but then once every couple months the therapist would come to daycare and observe and work with the child here but it was then looking at ways to better encourage the child to interact with the group. That was when I could ask for help I needed. I was always free to give my questions to the mom so she could ask them at the next visit she had.
The other negative thing about the therapies is you have to consider the other children in care and if the frequency and duration of visits will impact the group and how negatively such as preventing a trip to the park on certain days so you are home when therapist comes, can she only come during naptime, etc.
Ask the mom about previous daycare experience, interviews, what the most common concerns of potential caregivers seems to be as you want to be sure you are considering all angles of the situation and making an informed choice for everyone.
I agree that when you agree to work with a family dealing with a less then ideal situation you do often get one of the best most grateful families ever. Be prepared for them to need some emotional support along the way because it won't be smooth sailing for sure but the difference you can make in this child's life in even a few short months could mean so much down the road. At the same time do not let guilt or empathy sway you into taking on the child if you think his needs could best be met in a different situation. That also does the child an immense service.