Děti se venku cítí jako doma.
Nemělo by to mít podtext toho, že jsou to „děti ulice“, žijící venku, ale děti, pro které je být venku přirozené.
Kids feel outdoor at home – je asi správně, ale zní mi to i tak divně.
Outdoor is where kids feel at home – je možná lepší? Nebo napadá vás někoho ještě lepší obrat?

Pozor na slovosled: feel outdoors at home → feel at home outdoors.

„Outdoor“ je přídavné jméno:

• in wet weather you need to wear outdoor shoes, not flip-flops.
• he likes outdoor hobbies because he likes spending his time in the open air.
• they're planning (to have) an outdoor wedding so let's hope the weather's good on the day.

Příslovce je outdoors, outside, in the open air:

• flip-flops are no good for wearing outdoors when it's wet.
• He spends a lot of time outdoors on various different hobbies.
• they're planning (to have) their wedding outdoors so let's hope the weather's good on the day.

Your sentence: (Q: Where do kids feel at home?)

• Kids / children feel at home outdoors

And the cleft sentence version:

(Q: Why do kids like to be outdoors?)

• (Because) outdoors is where kids feel at home.

Some more ways of saying the same thing:

• Kids / children are in their element (when they're) outdoors.
• Kids naturally prefer to be outdoors/outside.

Also:

My kids are outdoor creatures, due to limited garden space we often need to go out, the woods and beaches always work for us, especially when it follows a crazy, losing-it moment. (mamainamuddle)

Children have a natural affinity to the outdoors. They are fascinated by it, attracted to it, and eager to explore it. This lasts until about age 14, at which point, unless parents have made a concerted effort to „provide their child with rich and repeated experiences in nature,“ they tend to lose it. (treehugger.)

Pravda, adj. x adv. – to mi uteklo. Děkuji za rozbor a varianty. 🙂