Differentiation under (measure theoretical) integral sign
Solution 1:
I found the answer to your question on p.142 of "Probability Theory: A Comprehensive Course, Ed. 2" by Klenke. Its called the Differentiation Lemma in that text. You've stated, almost word for word, the pre-conditions of that lemma. Here is a link that describes the conditions on exchanging integration and diffeentiation.