When do Aunt Polly and Mrs.Thatcher discover that their children are missing?
Aunt Polly and Mrs Thatcher realise this when they meet each other and Mrs Harper in church the day after the picnic. This is described in Chapter 31. Mrs Thatcher assumed that Becky didn't come home after the picnic because she stayed overnight at the Harpers, but Mrs Harper tells her she didn't. Things are made worse when Aunt Polly comes to ask whether Tom stayed with either the Harpers or the Thatchers overnight, as he hasn't come home either. This is the first time that people realise that Becky and Tom are missing, and fears are raised that they got left behind in the caves during the sightseeing expedition that followed the picnic.