Personally, I believe that the pregnant lady should get the seat mainly because she is carrying two people meaning her back has added weight which could cause signifiacant pain to her back.
The length of the journey could also be a key factor in deciding who should get the seat, because if the journey was short it should take much effort for the man to keep control of his two children he could also hold their hands to ensure saftey.
Other people could argue that, the man on crutches looked in more pain; however he has crutches to stabalise him alike to the elderly lady so he is unlikely to fall over. In addition, he has only suffered aa minor injury to his leg (as illustrated by the image presented) and it looks like it is nearly mended.
I strongly believe that the pregnant lady should get the seat beacuse it appears that she is far along in the pregnancy -6-7 months- so she is at a high point in her babys growth meaning she has the most weight over her pregnancy or nearly at the highest point.