For a variety of reasons, voices of animated children often end up belonging to real-life adults (see Bart Simpson and Tina Belcher), but back in the heyday of the Peanuts specials, Charlie Brown, Lucy, Peppermint Patty, and Linus were in fact all played by child actors.
The group was featured in a TV Guide spread in 1968 along with Bill Melendez—an adult actor who voiced the howls of Snoopy and drew the characters for the show. In the spread, we meet Peter Robbins (Charlie), Sally Dryer (Lucy), Gail DeFaria (Patty), and Christopher Shea (Linus), and get a few colorful details: For instance, Robbins thinks Charlie Brown is dumb and Dryer thinks Lucy’s a snob. Doesn’t get any more authentically childish than that.
Different actors would go on to play the roles in various iterations of the franchise, but if you want to properly match faces to voices, check out the always-great and seasonally appropriate It’s The Great Pumpkin, Charlie Brown.