All the frames in the comic strip could appear to be illuminated by the light of one little candle held either by Tom and Becky. The ceiling of the cave could be very high, and the figures of the two children could look very small in that spooky setting. It occurs to me that the candle would throw some spooky shadows of the children on the walls of the cave. Although Tom and Becky are lost, they are impressed by the beauty of the strange formations inside the big cavern. They keep traveling from one room to another, so there should be a succession of frames in which the formations are shown by candlelight. Mark Twain was familiar with the cavern he describes in the book, and he writes some beautiful descriptions of the stalactites that have formed from minerals contained in the continuously dripping water. I think the dialogue between Tom and Becky should be kept to a minimum because the balloons would detract from the beauty of the cave. You probably should show a few frames in which they lie down to sleep. Of course, you would also want to show the cave entrance with the beautiful outdoors when they finally find it. The dialogue can be taken directly from the book. The descriptions can also be derived from the book.