Based on your description:

"book I believe the MC’s name is Tilly or something like that. Her mom goes missing. She moves in with her grandparents who own a book store and discovers that she can enter the worlds in stories and she finds her mom trapped there in one of the stories. It’s a kids chapter book. I have it somewhere and can’t find it and off the top of my head can’t recall the name. Wanting to read it with my daughter. I’m sure it’ll come to me in the middle of the night and I’ll update."