You could always bus to AK/bus to RR but that wouldn’t be preferred. Are there still big lines for the Pop Skyliner station at 2hrs before park opening? I wouldn’t want to have to wait in that line for an hour or more with a pile of kids so the double bus trip might be preferred if HS opens at 10. What are the dates of your current planned trip?
Checked the skyliner sign for you today: AoA/Pop line and HS line both scheduled to open at 8 am and HS opened at 9. We rode from Riviera to Pop at 9, so our actual experience might not be helpful. There was an extensive line at CB and a shorter line at Pop. We told CM that we were going to Pop and they stopped the HS line and crossed us through. Later, when we returned everyone at CB was being funneled out of the skyliner and back up in one line together, since Epcot line was also open at that time.
So… what I think would happen for you, is there would be a line at Pop to get on the skyliner, but once at CB, I think they would send you straight across to Riviera line (since Epcot access wouldn’t be open yet and nobody else would be in line). If they didn’t, I’d recommend walking instead of waiting in the huge mostly HS line in order to get on the Riviera line.
Sorry I can’t help with what time to get in line. If I had to guess, I’d say the line at Pop at HS opening time was about 10-15 minutes, but you’d probably be leaving earlier than that. The line at CB was madness.
The skyliner runs from CBR to RR an hour before HS opens so RR people can get to HS but they were only letting you go towards RR if you had a reservation, since they didn’t want people lining up for Epcot opening 2 hours early.