When I was still teaching full time I often had students filled with great ideas who were unwilling to take the time or make the effort to write them out. That is not a criticism! It was most often NOT a reflection of laziness, but of frustration with the physical challenges of writing or the processing difficulties of moving from thought to paper. Students with challenges like this find it simpler, less frustrating, less anxiety producing, etc. to write the shortest possible answer they can possible do.
1. Do a trial first. Some night or weekend when there is more time and inclination take a question from a homework assignment or a past test. Ask your child the question out loud and have them answer it just out loud to you. If you find they know a lot more than what they wrote, you are on to something... go to the teacher.
2. Talk to your child's teacher. Show/Tell the teacher the difference between what you child typically produces for an answer and what they produce when freed of the responsibility of physically writing their answer. Ask if they are willing to accept your child "dictating" answers to questions. Years ago, I sometimes used cassette tapes for families who were willing and would then listen to the tapes to "correct" their "papers."
4. Meet again to plan. Tweak as needed or to use as a basic for discussing reasonable accommodations to enable more accurate assessment of your child's learning.
Make it clear and make it fun!! When my kids were younger I bought a plain bill-front cap from a craft store. Using markers and paint my child and I created a "Secretary" Hat. We even attached a couple of pencils and erasers to it for fun.
Whenever I was wearing the Secretary Hat, the child knew that I would write down exactly what he/she said with no changes or corrections or hints. If he wanted help or advice in revising etc. I took the hat off.
In time the hat was used sometimes by sitters, or by others helping at homework time. I later learned that one of the aides at school had made a similar hat for herself after hearing about it.
---And, by the way... another advantage is that the "boss" can pace or fidget all s/he wants while dictating.
Image credits: ipdc.co.id, affordablehousinginstitute.com
More or Less
7 hours ago