Been earning a couple hundred per year with a time-tracking/report-generating site for therapists-in-training. I think one of the big downfalls of the concept was that I chose too small of a niche to serve (marriage and family therapists in training in the US). The most tangible benefit from building the service was the coding knowledge gained which serves my freelance work very well. http://www.therapyhourstracker.com/
You should consider expanding this to include other types of therapy workers. I know from doing tech work amongst mental health professionals that most are required to log x amount of hours doing y before they can obtain licensing as a z. Since you have the platform and domain knowledge you should be able to add features to widen your target demographic.
