main.dart

void main(){
   ExampleReq exampleReq = ExampleReq(
     items: [
       Item(
         idProduct: 1,
         isWholesaler: 1,
         quantity: 12,
         variants: [1,2,3],
       ),
       Item(
         idProduct: 2,
         isWholesaler: 1,
         quantity: 12,
         variants: [32,3],
       ),
     ]
   );
  
  print(exampleReq.toJson());
}

model.dart

import 'dart:convert';

class ExampleReq {
    ExampleReq({
        this.items = const [],
    });

    List<Item> items;

    ExampleReq copyWith({
        List<Item>? items,
    }) => 
        ExampleReq(
            items: items ?? this.items,
        );

    factory ExampleReq.fromJson(String str) => ExampleReq.fromMap(json.decode(str));

    String toJson() => json.encode(toMap());

    factory ExampleReq.fromMap(Map<String, dynamic> json) => ExampleReq(
        items: json["items"] == null ? [] : List<Item>.from(json["items"].map((x) => Item.fromMap(x))),
    );

    Map<String, dynamic> toMap() => {
        "items": List<dynamic>.from(items.map((x) => x.toMap())),
    };
}

class Item {
    Item({
        this.idProduct,
        this.quantity,
        this.isWholesaler,
        this.variants,
    });

    int? idProduct;
    int? quantity;
    int? isWholesaler;
    List<int>? variants;

    Item copyWith({
        int? idProduct,
        int? quantity,
        int? isWholesaler,
        List<int>? variants,
    }) => 
        Item(
            idProduct: idProduct ?? this.idProduct,
            quantity: quantity ?? this.quantity,
            isWholesaler: isWholesaler ?? this.isWholesaler,
            variants: variants ?? this.variants,
        );

    factory Item.fromJson(String str) => Item.fromMap(json.decode(str));

    String toJson() => json.encode(toMap());

    factory Item.fromMap(Map<String, dynamic> json) => Item(
        idProduct: json["id_product"] == null ? null : json["id_product"],
        quantity: json["quantity"] == null ? null : json["quantity"],
        isWholesaler: json["is_wholesaler"] == null ? null : json["is_wholesaler"],
        variants: json["variants"] == null ? null : List<int>.from(json["variants"].map((x) => x)),
    );

    Map<String, dynamic> toMap() => {
        "id_product": idProduct == null ? null : idProduct,
        "quantity": quantity == null ? null : quantity,
        "is_wholesaler": isWholesaler == null ? null : isWholesaler,
        "variants": variants == null ? "" : List<dynamic>.from(variants!.map((x) => x)),
    };
}