I just received a rejection from an agent who said that my story lacked tension. My story is about 2 kids who go back in time, and must find a person (Pieter Bruegel) in order to come back home. In the meantime, they face various obstacles on their way to meeting this person. It's fast-paced and light. They have to overcome bed-bugs in one chapter. In other chapter, they get stuck working in someone's art shop while they should be out looking for Bruegel to get them back home. Each chapter has an obstacle, which the 2 friends work together to solve. There is character growth at the end of the story. It's not a poignant story meant to make you cry or have some terrific revelation at the end. It's meant to be a jolly good read.
Is there a way to have my kind of story have 'enough tension,' or is that an entirely different kind of story?
It's hard to say what kind of tension is lacking, without reading it. In the most obvious sense of the word, I would guess that the reader doesn't care about the bedbugs, because they won't ultimately prevent the kids from finding Pieter Bruegel. How long does it take them to find him? Five days? Suppose they were told they had three days to find him, or they could never get home. Now any obstacle that gets in their way could be disastrous, rather than merely annoying. Figure out the least amount of time it could possibly take them to find Bruegel, and give them even less. They'll be worried. They'll be nervous. They'll be tense. And so will the readers.