Thank you, both! I'll check both links. I haven't tried riding the hills alone, mostly because my son is insanely attached to me and we do literally everything together. If I tried to go for a bike ride and leave him home, he'd have a fit. Now that I think about it, he does go off with my husband occasionally, so I'll give it a try next time they're both gone. I have tried riding them with him in his baby seat (front mounted Yepp Mini) and it was impossible. I could ride down the hills, of course, but ended up pushing the bike back up them.
I'll do some more reading about the gears. I don't know that much about bikes in general (obviously), and on flat land that's never really been an issue.