Correction- Childrens Television Act of 1990. And it requires all digital channels to broadcast a certain amount of hours of E/I Programming.
Since digital television didn't exist in 1990 (the FCC was still looking at analog HD transmission systems at that time), I doubt that the Children's Television Act specifically addressed digital subchannels. Applying the 3 hours/week mandate to digital subchannels was an interpretation on the part of the FCC, and it went into play probably 15 years after the Act was originally passed.