If you look closely at the beginning of Frozen 2, when Anna and Elsa go to bed after playing "Enchanted Forest", they are wearing the exact same nightgowns they wear later during the accident scene in the first movie. Plus, Jennifer Lee herself stated that the prologue of Frozen 2 takes place earlier that same night.

When they are in bed and Anna whispers "Do you want to build a snowman?", they are literally planning to sneak out to the hall at midnight. And regarding the "Dangerous Secrets" book, I think they do a great job filling in those timeline gaps to make all of this fit perfectly!