Parse a nested JSON with Kotlinx.Serialization

I've been playing with Kotlinx.serialization, and I have been trying to parse a substring:

Given a JSON like:

{
   "Parent" : {
     "SpaceShip":"Tardis",
     "Mark":40
   }
}

And my code is something like:

data class SomeClass(
   @SerialName("SpaceShip") ship:String,
   @SerialName("Mark") mark:Int)

Obviously, Json.nonstrict.parse(SomeClass.serializer(), rawString) will fail because the pair "SpaceShip" and "Mark" are not in the root of the JSON.

How do I make the serializer refer to a subtree of the JSON?

P.S: Would you recommend retrofit instead (because it's older, and maybe more mature)?

import kotlinx.serialization.*
import kotlinx.serialization.json.Json


@Serializable
data class Parent(
    @SerialName("Parent")
    val parent: SomeClass
)

@Serializable
data class SomeClass(
    @SerialName("SpaceShip")
    val ship:String,
    @SerialName("Mark")
    val mark:Int
)

fun main() {
    val parent = Json.parse(Parent.serializer(), "{\"Parent\":{\"SpaceShip\":\"Tardis\",\"Mark\":40}}")
    println(parent)
}