May be worth poking around spare parts for brands like Frog / Isla Bike for options. My 3 1/2 y/o son's Frog 43 has some nice riser bars and some Tektro levers that work for his hands. I imagine they would fit the bill for your needs as well.
Try a local dealer and see if they are willing to source parts for you. It's a grey area so they may not. Better if your LBS is a dealer as they would be more likely to do you a favour.
May be worth poking around spare parts for brands like Frog / Isla Bike for options. My 3 1/2 y/o son's Frog 43 has some nice riser bars and some Tektro levers that work for his hands. I imagine they would fit the bill for your needs as well.