Adaptive tricycles for children with physical and cognitive challenges can be hard to find and even harder to afford. The adaptive tricycle we built for our son helped us get his legs moving as he recovers from a stroke that exacerbated the physical difficulties caused by his genetic disorder.
We took an off-the-shelf tricycle (Ybike Evolve) and attached a seatbelt, heel cups (foot plates), and a handlebar, all for about $25 in parts (plus some things that were just lying around the house) and with no tools except a drill, a screwdriver, and a small wrench.
Foot plates are fiendishly expensive if you buy them online. Instead, we bought a pair of oversized Mary Janes (girl's shoes) that fit over our son's shoes and braces. Check the clearance rack at your local big box store. These were about $5.
We cut the toes off and attached (with staples) a little extra Velcro to the Velcro strap so that it would be long enough to still attach over his bulky shoes.
We then drilled holes in the shoes and the pedals, and used countersunk bolts and nuts to attach the shoes to the pedals (nuts go on the pedal side). The countersunk bolts lie flat in the Mary Jane, so they don't catch on his shoes. This (and the next step) is where you need the small wrench. The bolts we bought were a little too long (they would scrape on the ground), so we cut them, but you could get around this by getting the right length bolts at the start.
These keep the child's feet in place if they do not have the strength or muscle control to keep their feet on the pedals themselves.
NOTE: Your child will not be able to pull their feet out. Please consider if this is safe for your child.