A girl forgot to do her homework and mum catches her out of bed:
「まる子っ、もう寝なさい」 Maruko, go to bed now!
「うん、もうすぐ寝るから」 Yes, I'll go to bed very soon (because)
I don't understand the function of から here. At first I thought it should be an unfinished excuse for why she is out of bed, but I thought that would be もうすぐ寝るが... Then I thought it could be "I will go to bed soon (rather than now) because (I have to finish my homework), but から is on the wrong clause for that to work. I'm out of ideas.