My Salvage Radiation (37 daily treatments over 8 weeks) were late in the afternoon - varied from 3:30 - 4:30 pm.
Afternoon worked very well for me.
And I am a morning bowel mover (to be direct).
Also took ADT during radiation therapy.
Whenever I am finished in the morning, I am "done for the day".
Additionally, I had some loose and multiple bowel movements in the morning during 2d half of radiation and needed to have a bathroom accessible. Could have been radiation or ADT or combination.
Did not have problems with "mostly empty rectum " for afternoon treatment.
My experience was the "mostly full bladder" half of the preparation was more challenging for me and most of the men.
Also, my two cents worth, I would not want to take my GD to my treatments. The bladder prep portion was stressful for me, and I had some distance to travel to treatments.
And while I would not hesitate to take a mature 9 yr old to my treatment if necessary by schedule or curiosity or for education, the waiting area was at times a sad and solemn place.
Good luck during your treatments. And enjoy the time with your granddaughter.