The stripes turned out okay. The edges aren't quite as crisp and clean as I'd hoped (a little paint bled through despite sealing and using the expensive painter's tape) but for now it'll do. I have an entire house to paint and once I'm done with the rest I might come back for a round of touch-ups.
I was hoping to find simple navy-blue Roman shades for the windows in his room but all I could find so far are those tie-up shades which I think are a bit too frou-frou for a boy's room. I might go for matchstick blinds and panels instead, hanging them up against the ceiling to create the sense of height and to balance out the effect of the hung ceiling.