For free web sources, try images.google.com, with searches like: mao children "picture books" site:edu (54 hits); mao "Yue Minjun" "children's books" (10 hits); and mao "cultural revolution" "picture books" (190 hits, but MANY are from publishers catalogs and library lists).
In a Google Web search, "mao era" "comic books" gets 207 hits, but "mao era" "comic books" site:edu may be too restrictive – there might be some useful .org and even .com sites among those first 207. Also, “mao era” will ALSO pick up “post mao era” and possibly even “pre mao era”(though none of those seem to show up on a quick scan of hits). Site:gov can help out too, although it may well introduce political bias of its own. Google Web search "cultural revolution" "picture books" site:gov gets 4 hits, and "cultural revolution" comic* site:gov gets 21.
USA.gov gets very different results that simply limiting Google to site:gov. For one thing, USA.gov does its own indexing and may go deeper into the sites, but also it includes MANY state government sites, including schools, museums, etc. The search: "cultural revolution" mao "children's books" gets 21 hits; "cultural revolution" “Children’s books” gets 62; "cultural revolution" mao "picture books" gets 8; and "cultural revolution" mao comic gets 54. Not many of these will be actual graphics, pictures, or comic books themselves – they are papers and reports talking about them, and may be good if you are doing more detailed research.
Searching Google Directory gets you to Cultural Revolution, which might have some useful examples. The trick here will be to check the home pages and any “About Us” links to see who has compiled the information. Any lack of cited references, or difficulty in tracing “who says this”, is a cause for concern – even if you are not writing for a peer reviewed journal. Just a brief personal or institution name can be fairly quickly checked in scholarly indexes, but it is nice if the site itself does some of that documentation for you.
As to scholarly or detailed research, if you are near a large public or academic library (or have remote access to journals and databases as a student, staff member) there will be some articles in journals about this. In the full text journals, searchable database JSTOR, the full-text search: "mao era" AND "picture books" gets 4 hits; mao AND "picture books" (41 hits) is a better bet – to miss fewer potential good sources. Mao AND "children's literature” (173 hits) may be too broad a search – at least at first. I am finding very little in other commonly used databases – Academic Search gets 5 hits for mao and comic; cultural revolution and children and literature gets 8 (mainly book reviews), and cultural revolution and comic* gets only 2. Even changing the default “select a field”(title, abstract, subject and author) to “Full Text” is not too promising. Cultural revolution and picture books (34 hits) seem to be largely unrelated – maybe Mao is mentioned only in passing, or is an entirely different Mao.
background on young pioneers
I was doing research a few years ago on this very topic. The "young pioneers" were also known as "ying xiong" or heroes/role models. The most prominent of them was Lei Feng, but there were many others whose actions were supposed to inspire young children. Images and writings about these people were everywhere, including posters and school readers. These school readers are only in specialized education publishing libraries. However, many children's books about the role models are still sold in China today. For some background and images of these role models, there is Stefan Landsberger's website on propaganda posters in China. You can probably come up with more search results with "Lei Feng," "heroes," or "role models" and "Cultural Revolution."