In this sentence we have Lo +adverb + que. The adverb here is "rápido", and adverbs don't agree in gender or number, they are invariable. I know that you may think that rápido can also be an adjective, e.g "coches rápidos", but in this case it is not acting as an adjective.
If we have a look at the sentence again: "¡Lo rápido que han hecho los chicos los deberes!, you can ask ¿how did they do their homework? answer: "quickly" -> adverb rápido =rápidamente.
However, it would be different if we said: "¡Lo rápidos que son los niños!" = How fast the kids are! Here, rápidos would be acting as an adjective for "niños", therefore they need to agree.
There is a note in the lesson explaining how you can spot when they are adjectives or adverbs.
I hope this makes it clearer for you to see the difference.
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