Hello: My daughter has the same problem. The answer for her was to sit any way she wanted. That meant laying on the floor on either her back or belly,or on her back with her legs on the sofa. As most elementary school homework is reading and math this was no problem. Also, my daughter started a new school district last year and had a seating arrangement I had never seen before. They had multiple seating choices in each class. They could sit on the floor, a yoga ball, a beanbag chair with a lap desk, and several other options.
Good luck with your kiddos!