add possibility to filter child elements

There might be a better way to do this, but there's an ordering bug https://gitlab.com/arkindex/backend/-/issues/43 that causes some elements to be skipped in the workflow. When filtering the children by the type this problem doesn't seem to appear.

Even though it is not currently possible to set the type in the frontend, I could at least do it via API after this will be merged.

Ref #15 (closed)