Hi there. I would definitely say that the Vit D deficiency is a cause of your child's forgetfulness. I had symptoms of forgetfulness which was extremely scary. I thought it might be the early onset of alzheimers. I also felt very tired and had trouble sleeping as well as sore muscles. My Drs initially thought it was an issue with Iron. After some research on the internet I found out about Vitamin D. My level was low 19. After a few months of supplementing I felt much better and not so forgetful. After the 6 months supplementation of 50000 IU once a week, I would randomly take the supplementation as too much can be poisonous. Last year I felt the same tiredness and forgetfulness. I didn't think it would be the Vit D, however after blood tests my Vit D levels dropped downto 20.
I am now back on supplementation. My new Dr wants to take it up to between 50-70. I would suggest you monitors your child's Vit D regularly to prevent a recurrence, and also get it into the optimum range.