I think that that would be the Dragonlance Campaign Setting. Cover shot of the book:
The only mention of Kender and religion in it is as follows (p. 30):
Though kender recognize all the deities (to avoid hurting any god's
feelings), they hold Branchala, Chislev, Mishakal, and Gilean in
highest regard. Reorx is viewed as a grumbling but benevolent
grandfather, but kender do not specifically praise him.
The book further mentions that because of Tasslehoff, some kender also regard Fizban (one of Paladine's avatars) as a revered deity.
I personally am tempted to go with Branchala as the primary deity because the description of him fits the most with my idea of kender. The deity's description (p. 121) begins as follows, for instance:
The Bard King, Branchala, brings joy to the people of the world
through melody and merriment with his unending song of life.
The idea of a god whose main goal is to make people happy seems to fit really well with the general wanderlust/careless nature of the Kender.