The Youngest Templar serial includes characters who adult readers will recognize as young Robin Hood and Maid Marian. Long before they appear, our young protagonist has a brief encounter with a man named John Little. "But you should call me Little John," he says. "Everyone else does."
Could anyone read that and not know who was being referred to?
I don't know. Are kids these days familiar with Robin Hood? There are have been lots of books on the traditional Robin Hood story over the last few decades, as well as some variations. A graphic novel was published in Australia this summer, and a Muppet Robin Hood comic sold out in one week earlier this year. (The publisher did a second printing.) You'd think some kids would have some familiarity with the story, though I don't think they need to recognize who the characters in The Youngest Templar are supposed to be in order to enjoy them.
I think Robin Hood has traditionally been treated as a children's story, unlike Sherlock Holmes who, nonetheless, turns up in a lot of kids' books. So kids may readily get Robin references. Or, as someone at Guys Lit Wire suggested, Robin Hood may be an iconic character "that people think they know without bothering to read the actual stories they appeared in."