This is a question about personal reaction to a text, also called reader-response theory. I was not surprised the first time I read the novel that the children would go off with Mrs. Who, Mrs. Whatsit, and Mrs. Which, because L'Engle sets her story up to make it seem completely plausible.

First, we are educated to trust these three women, largely because Charles Wallace does but also because they come across as kindly and unthreatening individuals who have a special interest in the children. Further, they offer a way to find Meg and Charles Wallace's father, which the children want very much to do. Finally, Meg, Charles Wallace, and Calvin are children or adolescents, and like most young people, they are up for an adventure.

They go off with these women in large part too because the elderly threesome whisks them away without asking. Charles Wallace tells Calvin that they are "going," and when Meg asks where, he responds:

I don’t know exactly, Charles Wallace said. But I think it’s to find Father.

The children are fine with going, however, because they instinctively trust the three women.