Documentation for 9yr old Spanish national traveling to UK

We are a Spanish couple planing a trip to London with our 9 yr old son next month. I assume there is a minimum age for which you need to have your own documents, but I am not sure what that might be, and can't seem to find it online.

I know you can travel using your national ID card, but in Spain you are not required to have one until you are 14 yr old.