Registration for Spartan Kids Races are available on race day at the venue. Look out for Spartan Kids Race signs at the registration tent and ask to register your child. You will be given a waiver to sign, a bib and be communicated the start times. This is available for kids from 4 to 15 years old.
Of course you may also register the little Spartan online: https://www.spartanrace.gr/en/race/detail/4874/overview?filter=kids&index=3