This isn't Janice, this is Lila.
The new Peanuts special, It's The Small Things, Charlie Brown, recently came out. (my apologies for spoilers if you haven't seen the special) It follows Sally as she befriends a little dandelion and vows to protect it from getting trampled. Charlie Brown finds it ridiculous that Sally is protecting "a measly little weed" in the middle of their big championship baseball game. He tries to recruit Lucy to convince Sally get rid of the flower, but instead, she and the other Peanuts kids create a sit in on the mound in honor of the dandelion and try to beautified the baseball field. You'll noticed that there are many other minor Peanuts characters, such as Lydia, Lary, Loretta, Naomi and many others that are in the crowd. If you look closely, you'll see a little girl with blonde hair with a bow in her hair, but it's not Janice.
I just want to make one thing clear, this is not Janice, this is Lila. Lila was one of the original Peanuts characters from the comic strip. She has long straight blonde hair that goes all the way down to her shoulders with a headband on top. She made her first, and only appearance in a strip from August 24, 1968. Later on, it was revealed that before Charlie Brown adopted him, she was Snoopy's original owner. She adopted him from the Daisy Hill Puppy Farm and she would spend almost all her time playing with the puppy. However, it was revealed that her landlord didn't allow dogs at her apartment complex and she sadly had bring Snoopy back to the puppy farm. It doesn't even end there; in the movie, Snoopy Come Home, (my apologies for spoilers if you haven't seen the movie) when Snoopy does return to her, he finds out that she owns a cat and that her apartment still doesn't allow dogs. He then goes back to Charlie Brown, leaving Lila heartbroken.
Janice is a character from the TV special, Why Charlie Brown, Why. She's a little girl with short blonde hair with a bow on top who loves to play on the swings. She is very good friends with Charlie Brown and Linus. Linus has a somewhat of a crush on her as enjoys pushing her on the swings and hanging out with her. However, she's suddenly diagnosed with leukemia and must go under chemotherapy to get cured. Linus becomes her defender who protects her from bullies and sticks up for her. In the end of the special, Janice goes through remission and grows her hair back and play on the swings with Linus again.
So you see, Lila and Janice are two totally different characters. The differences are that they appeared in different pieces of media, have different hairstyles, character backgrounds, etc. In conclusion, I hope that clears that up. Have a very nice day.