To do a thorough search for scores in UMD Libraries, do not limit your search to MSPAL. A substantial number of scores are in microfilm collections at McKeldin Library.
Some scores not owned by UMD are in the USMAI Libraries system. In the catalog, selecting "USMAI (all campuses)" broadens your chances of finding what you need.
For example, doing an all-campus search on "children's" as subject word and "scores" as format will yield 407 hits. Doing the same search with just UMCP as location yields only 373 hits and searching just in MSPAL yields only 254 hits.
A note on juvenile scores: although USMAI has many juvenile scores shelved in the Library of Congress Classification System's M series, at UMD's McKeldin Library (2nd floor) and some USMAI campus libraries, juvenile scores have call numbers in the Dewey Classification System's 780 series. You can do a basic search on call number 780 in "Catalog -- USMAI (all campuses)" to browse these scores.
Since 2004, scores deposited as UMD theses and dissertations are available online in DRUM. Hard copies prior to that date are available for study in MSPAL and the Maryland Room in Hornbake Library. Full texts from 1997 to the present are also available in ProQuest Digital Dissertations & Theses Global, where dissertation and thesis scores from other universities are available to UMD Libraries researchers.