Are "gratitude" and "appreciation" suitable CC-CEDICT definitions for 感激之心?
"感激之心" literally is "the heart that is grateful/appreciated/thankful". And I think it should be kept that way, which is a phrase not a word. Its meaning can be derive from words in the phrase easily. I don't think it should have its own entry.

"恻隐之心" has its own entry, probably because "恻隐" is seldom used alone now. "恻隐之心" has become a set phrase, or a word.

"赤子之心" is different, it means "the heart of ...", and it has its own figurative meaning which is different from that of "赤子". So it got its own entry.