I'd say that the first day of potty training was a total success! We got 5 pees on the potty and Phoenix knows what we want her to do when we sit her on the toilet. The time where she sits and fiddles around before peeing has dramatically reduced from about 15 minutes to 1-2 minutes. This tells me that 1. She has figured out that she is supposed to pee when she sits on the potty and that 2. She is gaining better control of using her muscles. Both of these accomplishments are really good news in the potty training process!
I have read a lot the the low muscle tone can really delay potty training in kids with DS. Having low tone can mean that the way that our kids feel their muscles can be reduced. Phoenix is now at the upper end of the age range for potty training of typical kids. So the fact that she is showing interest and some level of control is very, very good news.
Will she be potty trained for when she goes to preschool in the fall? Who knows? But 5 pees in the potty is something to celebrate!