While comic books have long been considered as something just for kids, that is, of course, not true. However, that does not mean that kids do not read comics and that there are even comics written specifically for children ranging from wordless comics like Owly to those simple ones for beginning readers to those aimed specifically at an elementary school-aged and/or "tween" audience.
What makes a comic or graphic novel "juvenile?" Often it's same elements that make a work of fiction (or even non-fiction) juvenile – including things like reading level and a content lacking to many "mature" elements. It also should be of interest to that age group, generally six to twelve years old. You may have a graphic novel in your adult collection whose content is totally "G" rated but whose subject matter would not appeal to younger readers. Having graphic novels that are cataloged as "juvenile" (or "kids" or whatever other term you may use) can help younger readers to find them, even if you have chosen to shelve them intermixed with graphic novels for other ages. And while those cataloged as young adult, and in some case adult, can be fine for younger ages (especially the tweens), it is nice to be able to point to a book and say that it for kids.
Choosing what graphic novels to purchase for the juvenile can often be difficult with the large range that is out there. It has been said that the age average comic book/graphic novel is at least in the late 20s and some mainstream comics are written to appeal to that demographic. And while it's often easy to tell which books a definitely NOT suitable for younger ages, as with "regular" books, there sometimes is a grey area as to what can be cataloged as a "juvenile" item as opposed to Young Adult.
In some cases it can be easy as there are particular titles and even publishers that are well known as being "kid friendly." The titles from Toon Books are perfect for those children who are beginning to read on their own, while titles from Scholastic's Graphix line have become hits in Elementary School book fairs, especially with Jeff Smith's Bone series and Raina Telgemeier's Smile and Sisters. Lerner and Capstone are among the publishers of children's non-fiction who also produce graphic novels, both in their own name or with imprints such as Stone Arch and Graphic Universe (though the later has begun to include YA titles).
With some of the larger companies you often can tell just by looking at the work. Marvel's "Marvel Universe" titles (as well as their older Marvel Adventures and Marvel Age lines) has the familiar characters in kid-friendly digest-sized books, and DC has titles such as Tiny Titans and series based on various animated programs. And of course both companies have their ratings systems that show what titles are recommended as being fine for younger readers. Other publishers, including producers of manga have their own rating systems, usually with some variation of an "All Ages" rating.
If you want to compile a list, the vendor that your library uses can be helpful. Often their websites will let you filter a search by the suggested age range of the work which you can use with a "graphic novels" keyword or a publisher-specific search. They may even create lists of "Kidsafe" titles that will help with your selection.
There are also a number of review sites that will be about juvenile graphic novels or dedicate a section to them such as No Flying No Tights. And Not only does the print edition of School Library Journal include reviews on graphic novels for childrens and tweens (as well as tweens) but their online blog Good Comics For Kids has reviews of kids's comics and graphic novels on an almost daily basis. The reviews are written by librarians and also include articles, interviews, and more.
If you are still not sure about a title and if falls into that "grey area" between juvenile and young adult then you can make the final decision once you get the title in your library. If your cataloging and/or processing is done "in-house" then you might be able to delay it until you review the material. If not, hopefully you have a way to change it after the fact. In that case it may be better to make young adult the "default," as it is better to have something for a younger age in an older area than vice versa. In addition the position of the line between juvenile and young adult can be different depending on the nature of your community. Of course if you are purchasing for an elementary school library, cataloging "upwards" may not be an option, so if you are still unsure, it might be best to wait to the book is out and try to browse a copy at a local bookstore or comic shop. A review copy, either in book or electronic form might also be available to you.