4 to 13 year olds can take part without specific training. There’s even Special Spartans heats for kids with special needs. Kids simply need a desire to run outdoors and have fun. They need not be fearless or fit; both those things will develop after taking part! There are kids of all abilities and mindsets out there, and they all get to call themselves Spartans.
Comfortable clothes suitable for exercise and sturdy footwear is all that’s needed - just be aware that your kid’s gear will get wet and muddy! Pack a hat and sunscreen if it’s hot, extra layers if it’s cold, and a change of clothes and a towel for afterwards regardless of conditions. After-race refuelling of food and drinks can be purchased in the festival.