Where do Meg and Charles Wallace meet Calvin? Who are they originally looking for?

Meg and Charles Wallace meet Calvin in the woods near Mrs. Whatsit's house. They are in search of Mrs. Whatsit. Charles Wallace wants to talk to her for several reasons. He wants to asks her about the "tesseract thing" because he saw that it upset their mother, and if their mother is upset, he knows the "tesseract thing" must be important.
Second, he wants to warn Mrs. Whatsit about gossip in the village over some stolen sheets. He fears Mrs. Whatsit might have taken them without realizing she was doing anything terribly wrong. He wants her to be aware that there might be some retaliation and so to be on her guard.
When he initially sees Calvin in the woods, Charles Wallace reacts to him with suspicion because he fears that Calvin might have mischief in mind against Mrs. Whatsit.