Mysterious Monsters Series by David Michael Slater

The Mattigan kids don't believe in things that go bump in the night. After all, their dad is famous for proving such things are impossible. But, when their long-lost Grandpa Joe shows up with his Mysterious Monsters journal, begging for help, the siblings find themselves drawn into a search for Bigfoot. Along the way, they'll have to deal with meddling babysitters, suspicious psychics, a YouTube disaster, and their furious father... moreThe Mattigan kids don't believe in things that go bump in the night. After all, their dad is famous for proving such things are impossible. But, when their long-lost Grandpa Joe shows up with his Mysterious Monsters journal, begging for help, the siblings find themselves drawn into a search for Bigfoot. Along the way, they'll have to deal with meddling babysitters, suspicious psychics, a YouTube disaster, and their furious father. To solve this mystery, Maddie, Max, and Theo must rethink what's possible — and make lots of peanut butter and banana sandwiches. less