Our son had been getting Methylphenidate (Quillivant) in the mornings, and we had a similar cliff effect. We have been giving our son a booster that would take us from the end of school until about 7:00pm. Worked wonders for his behavior and ability to do homework, etc. He had been on Quillivant, then Quillichew. Just started him on a longer lasting 36mg of Concerta, which seems to be a good, single 12 hour dose. So preferable to the four a day he had been getting. He is 8yo and about 50lbs.
We have the nurse at our 8 year old’s school give him his afternoon dimethylphenidate at 3:00pm every day. This gets him through after school activities, homework, etc. and wears off around 6:30pm. You will of course have to fill out some paperwork to allow them to administer the medication and provide them the medicine, but it is well worth it.