How to use System.Linq.Expressions.Expression to filter based on children?

Well, if there is a 1:1 relationship between parent and child (unlikely, but the example seems to imply that) then you could do it like this:

  return db.Parents
  .Where(parent => parent.Status == 1)
  .Select(parent => parent.Child)
  .Where(filter)
  .Select(child=> child.Parent);

Otherwise it will be hard.

You could do it with dynamic linq but that is probably overkill.

You could generate your expression tree manually, but that is also quite complicated. I have not tried that myself.

As a last resort you could of course always call yourQuery.AsEnumerable(), this will cause linq-to-sql to translate your query into sql up to this point and perform the rest of the work on the client-side; then you can .compile() your expression. However you lose the performance benefits of linq-to-sql (and compile() itself is quite slow; whenever it is executed, it calls the JIT-compiler):

  return db.Parents
  .Where(parent => parent.Status == 1)
  .AsEnumerable()
  .Where(parent  => filter.Compile().Invoke(parent.Child))

Personally I'd just define the expression twice, once for child and once for parent.child:

   Expression<Func<Child, bool>> filterChild = child => child.Status == 1;
   Expression<Func<Parent, bool>> filterParent = parent => parent.Child.Status == 1;

Might not be the most elegant, but probably easier to maintain than the other solutions

If you want to combine expressions and still be able to use linq-to-sql, you may want to have a look at LinqKit. It walks inside your expression and replaces all the function calls by their contents before the sql conversion.

This way you'll be able to use directly

return db.Parents
       .AsExpandable()
       .Where(parent => parent.Status == 1 && filter(parent.Child));

You can try this:

var compiledFilter = filter.Compile();
foreach (var parent in db.Parents.Where(parent => parent.Status == 1))
    if (compiledFilter(parent.Child))
        yield return parent;

It requires you to pull all of the parents, but unlike @HugoRune's solution, it doesn't require a 1:1 relation of Parent:Child.

I don't think this will be useful for your situation because of the different types involved, but just in case, here is an example of how you can combine Expressions: How do I combine LINQ expressions into one?

Edit: I had previously suggested using Compile(), but that doesn't work over LINQ-to-SQL.