Dragonbreath 4: The Lair of the Bat Monster by Ursula Vernon

Danny Dragonbreath is hoping to see lots of cool bats when he and his best friend, Wendell, visit Danny's cousin, a bat specialist in Mexico. But he isn't expecting to get up close and personal with a giant bat monster, who kidnaps Danny and carries him off to her lair! Now nerdy Wendell will finally get a chance to be a hero... but only if he can hunt down the bat monster before she permanently adopts Danny as her bat monster baby.