A deeper answer which is the conclusion i came to, and i have found your posted question while searching for confirmation, could be to do with the BFG himself. When the other giant enters with a "booboo", during the scene he (i think either bloodbottler or fleshlumpeater - a discrepancy here between the 1989 animated and current version of the film) says something about a pet and a desire to eat it. Perhaps the BFG had a previous captive, why else would he have a basket of clothes. It also explains why he is so protective of Sophie as the logical conclusion for the whereabouts of the previous child would be dead. Seeing as the BFG has a large interaction with children and dreams it could be a logical summary.
