示例数据：

public class Data
{
    public int Id { get; set; }
    public List<int> ChildIds { get; set; }
}
var data = new List<Data>
{
    new()
    {
        Id = 1,
        ChildIds = new List<int> {123, 234, 345}
    },
    new()
    {
        Id = 1,
        ChildIds = new List<int> {123, 234, 345}
    },
    new()
    {
        Id = 2,
        ChildIds = new List<int> {678, 789}
    },
};

使用foreach实现：

var dict = new Dictionary<int, int>();
foreach (var dataItem in data)
{
    foreach (var child in dataItem.ChildIds)
    {
        dict[child] = dataItem.Id;
    }
}

1、使用Linq的SelectMany方法实现

var data = new List<Data>
{
    new()
    {
        Id = 1,
        ChildIds = new List<int> {123, 234, 345}
    },
    new()
    {
        Id = 1,
        ChildIds = new List<int> {123, 234, 345}
    },
    new()
    {
        Id = 2,
        ChildIds = new List<int> {678, 789}
    },
};
ILookup<int, int> idLookup = data
    .SelectMany(d => d.ChildIds.Select(c => (Id:d.Id, ChildId:c)))
    .ToLookup(x => x.ChildId, x => x.Id);
Dictionary<int, int> dict = idLookup.ToDictionary(x => x.Key, x => x.First()); 

或者

var data = new List<Data>
{
    new()
    {
        Id = 1,
        ChildIds = new List<int> {123, 234, 345}
    },
    new()
    {
        Id = 1,
        ChildIds = new List<int> {123, 234, 345}
    },
    new()
    {
        Id = 2,
        ChildIds = new List<int> {678, 789}
    },
};
var dict = data 
        .SelectMany(d => d.ChildIds, (data, childId) => new {data.Id, childId})
        .Distinct()
        .ToDictionary(x => x.childId, x => x.Id);
dict.Keys
        .ToList()
        .ForEach(k => Console.WriteLine($"{k} {dict[k]}"));

或者

var list = new List<(int Id, int[] ChildIds)>()
        {
            (1, new []{10, 11}),
            (2, new []{11, 12})
        };
        var result = list
            .SelectMany(pair => pair.ChildIds.Select(childId => (childId, pair.Id)))
            .ToDictionary(p => p.childId, p => p.Id);

2、使用Linq语法实现

Dictionary<int, int> dict2 = (from item in data
                                from childId in item.ChildIds
                                group new { item.Id, childId } by childId into g
                                select g.Last()
                                ).ToDictionary(kv => kv.childId, kv => kv.Id);

或者

Dictionary<int, int> dict = (from item in data
                             from childId in item.ChildIds
                             select new { item.Id, childId}
                             ).Distinct()
                             .ToDictionary(kv => kv.childId, kv => kv.Id);