707 characters (excluding the comments). Looking forward to seeing a more efficient option. 

/**
 * @Intent - Fish Detector
 * @description - Identify those that own fish and are under 20
 */
foreach device in network.devices
let persons =  { name: "Tim", age: 21, pets:  "dog", "mouse", "fish"] },
               { name: "Mark", age: 19, pets:  "dog", "fish", "cat"] },
               { name: "Nina", age: 18, pets:  "snake", "lizard"] },
               { name: "Leroy", age: 24, pets:  "fish"] },
               { name: "Sally", age: 11, pets:  "dog", "fish"] }
              ]
foreach person in persons
foreach pet in person.pets
where person.age < 20 && pet == "fish"
select distinct { Name: person.name, Age: person.age, Pets: person.pets }

707 characters (excluding the comments). Looking forward to seeing a more efficient option. 

/**
 * @Intent - Fish Detector
 * @description - Identify those that own fish and are under 20
 */
foreach device in network.devices
let persons =  { name: "Tim", age: 21, pets:  "dog", "mouse", "fish"] },
               { name: "Mark", age: 19, pets:  "dog", "fish", "cat"] },
               { name: "Nina", age: 18, pets:  "snake", "lizard"] },
               { name: "Leroy", age: 24, pets:  "fish"] },
               { name: "Sally", age: 11, pets:  "dog", "fish"] }
              ]
foreach person in persons
foreach pet in person.pets
where person.age < 20 && pet == "fish"
select distinct { Name: person.name, Age: person.age, Pets: person.pets }

 

The only tweak I see is to not involve network.devices. 

I replaced foreach device in network.devices with foreach x in 0]
 

And the usual things to do to minimize characters (not including comments in the NQE ….
 

foreach x in c0] let s = l{ n: "Tim", a: 21, t:  "dog", "mouse", "fish"]},{ n: "Mark", a: 19, t:  "dog", "fish", "cat"]},{ n: "Nina", a: 18, t:  "snake", "lizard"]},{ n: "Leroy", a: 24, t:  "fish"]},{ n: "Sally", a: 11, t:  "dog", "fish"]}] foreach p in s foreach t in p.t where p.a < 20 && t == "fish" select distinct {Name: p.n, Age: p.a, Pets: p.t}