I see him as a definite Fi dom, with a big emphasis on making the child feel accepted, valued and safe. His approach seemed more to be about making the world less scary for the child, rather than having the child prepare and conform to the world. He seemed more about acceptance and reassurance, and less about obligation and responsibilities (although clearly there was some of both). He was also markedly anti-commercials and anti-marketing, which also seems a good fit for an Fi-dom, since Fi-doms are often notably unmotivated by commercial concerns.
As far as ISFP vs INFP... his major was music composition, which would lead one slightly towards ISFP, although certainly composition can be approach theortically. However, he was also ordained a presbyterian minister, which I see as a better fit for INFP; while I know ISFPs can and do help the people in their lives, their help is often much more a practical, hands-on kind of helping rather than a more abstract, verbally based kind of assistance.
Also, I think Mr Rogers was directly addresses the inner fears and concerns of children as direct concepts. He seems more verbal than most ISFPs I know, who tend to express themselves less verbally and more physically and artistically.
Still, I think it's a tough call, since one could argue that his show was his art through which he expressed himself musically, verbally and visually. Still, most MBTI resources list him as a quintessential INFP, so I don't see a whole lot of reason to doubt it.