The SALT (Speach and Language Therapist) will more than likely want you to be present, as you will be able to calm your DS better, if he becomes tricky to handle, plus the SALT can ask you questions about his language usage at home.
You could post a question in Special Needs about what happens when a child goes for speech therapy, so you know what to expect at least for the initial few sessions.
A childminder is more than likely to have other children to care for... it isn't really nice for them to be going along to a therapy session. If the childminder didn't have any other children to care for, then perhaps they would be prepared to go, but I feel only once you have been with your DS for a few sessions and the therapist is happy for you not to be present.
Working parents have rights to time off for dependants.