You don’t say how old your child is, so I’m not sure how much grammar he has already studied. But, the comparative form and the superlative form relate to adjectives. In the word you supplied, thin is an adjective. If you are talking about one thin thing, then you use the word ‘thin.’ For instance, “Mary is thin.” But if you want to compare (think comparative here) two people by using this adjective you would not say, “Mary is thin than Alice.” Instead you could say, “Mary is thinner than Alice.” Usually, when the adjective ends in a consonant, you double the consonant and add er to make it a comparative adjective; and everyone would understand that you are comparing two things. You use the superlative to compare more than two things. So, taking the same adjective, let’s pretend there are four people involved. We would say, “Mary is the thinnest of her four friends.” We don’t need to name them; we simply need to tell the listener or reader that more than two people are involved. Usually this is done by doubling the consonant and adding est to the adjective. Example: fat,fatter, fattest. Not all comparative and superlative forms are formed this way, but this is a good place to begin.