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Is this maybe part of the goosebumps series or another novel by RL Stine?
I'm pretty sure it's not by RL Stine, and I'm certain it's not part of the goosebumps series
It sounds sort of like The Borrowers by Mary Norton, though you didn't say how much smaller the father and daughter are.
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Flowers in the Attic?
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I've had a look at both The Borrowers and Flowers in the Attic, there are similarities but it's not either of those.
A boy (i think at oldest 13) lives with his grandmother. His bedroom is on the top storey of their house and there's a door from his room into the attic. At some point, he discovers that there are two people (i believe a father and daughter) living in there. From memory, they aren't human and are significantly shorter than humans are. The boy befriends them while trying to keep their presence a secret from his grandmother, though it's insinuated she knows that they're there and has for some time.