Try using some blankets as props. A blanket roll (as big as needed) under your hips during child's pose will make some space between the hips and calves, and your knees won't have to bend as much. Another blanket under your knees will also provide a nice cushion.

Pigeon pose can be done on your back as a "figure four stretch." Then you can focus on keeping your knee at a 90 degree bend while stretching the hip on that side.

As an alternate to pigeon and child's pose you can lie down and hug your knees toward your chest, with your hands resting on your shins or the backs of your thighs.