I have been in your shoes. How you handle it depends on the type of relationship you have with the current provider...for instance, is it a single provider (in home care) or is it a center? Typically, the in-home daycare provider develops a much more personal relationship with the families so you may want to do say more than just just give notice. In that case, you may want to have your notice in writing but verbally tell her before you hand it to her. You can elaborate more if your relationship deems it.
Either way, what you put in your written notice should be something like "Johnny has been accepted to a spot a church based program which will offer him a more varied experience. His last day in your care will be March 15th. Thank you for providing excellent care, as we are very happy with that. Please feel free to use us as a reference."