Final answer:
The word ruefully means expressing regret or sorrow in a slightly humorous way. In the context given, it shows Samuel's regret over not being able to play soccer due to his family responsibilities.
Step-by-step explanation:
The word ruefully in the sentence "Oh, you know, another day in paradise," Samuel answered ruefully. "I've got kid duty with a third-grader who refuses to eat her snack and a baby who probably won't take her nap.
I know I need to share responsibility for my sisters, but all I can think about is being on the soccer field with you guys." means expressing sorrow or regret, usually in a slightly humorous way.
Samuel uses the word to convey his mixed feelings of responsibility and regret that he's not able to join his peers on the soccer field. The use of ruefully suggests that while Samuel acknowledges the duties he has towards his family, he can't help but feel a pang of remorse about the personal activities he is missing out on.