The mummies are representing the youth of kids. Holden wants to protect the young kids from the real world since he believes they aren't ready. When waiting for phoebe he helps 2 young boys find the mummies since he wanted to help them that since they died they never grew up.
“While I was waiting around for phoebe in the museum, right inside the doors and all, these two kids came up to me no asked me if i knew where the mummies were” (223).
“That way they could be buried in their tombs for thousand of years and their faces wouldn't rot or anything” (224).
Holden enjoyed going to the museum. The thing he liked best was how nothing changed over time. When Allie was alive he used to go with Holden. So when Holden goes to the museum now it reminds hum of Allie and how nothing has changed since they went when he was alive.
"The best thing, though, in that museum was that everything always stayed right where it was. Nobody'd move" (135).