A caption is any writing that the child chooses to write to accompany a picture, it doesn't have to be two words and I wouldn't really accept that from any but the very challenged. I would ask all children to think of their own caption and try to represent it in sounds. For the sentence 'I can see a brown cat' some children will just hear a few sounds and will write eg ' I s b ct' this is fine and should be praised. Extend this by asking if they can hear a sound at the end of 'brown' or if they can remember how to represent the 'ee' sound. Definitely mention using capital letters and a full stop at the end. Also remind about using finger spaces. Maybe have a big laminated finger prepared so that you can write your own caption and demonstrate how you leave a gap between each word. Good luck for Monday.