Perhaps, you could start by looking at the writing plans that your daughter is producing – is there enough jotted down in these to allow for a reasonable amount of writing? Is the plan well structured and broken down into paragraph plans?
Secondly, take a very close look at your daughter’s sentence structures – is she using enough compound and complex sentences? Can she make simple sentences longer by adding phrases or combining sentences?
Encourage your daughter to expand her sentences by greater use of adjectival and adverbial phrases. These will not only make the writing much more interesting but will also expand the length. In the same manner, the use of imagery for descriptive writing (similes, metaphors, personification etc.) will make the writing more interesting and expand the length.
Part of the challenge with timed writing tests is the that your child will feel rushed and therefore not write as much as they are able. It may therefore be a good idea to start by not timing her writing, and allowing her enough time to write enough, and then gradually bring the time down.
Hope that helps.
Anybody else got some helpful ideas they can add to this?