When calling .FindById(), it’s a relatively simple task, to have it include child properties explicitly. Simply add a lambda expression.
This will ensure that the ChildListis populated and not null.
But what if you need to ensure a GrandChildren entity is populated as well? Writing this won’t work because ChildList is a collection.
What you need to do, is use .Select()
myRepository.FindById(1, x=>x.ChildList, x.ChildList.Select(y=>y.GrandChildren) );