I highly doubt anyone in line is going to notice 2 adults getting on a ride without kids. For all they know, the kids in line around you might be yours but just riding in a different seat. And the CM won't say anything. Go and enjoy. Who cares if it seems like a kiddie ride. Most people when they're in Disney are just really tall kiddies
anyway. I know I am.
And Mickey's Philharmagic was really good IMO. If you like Disney movies and Disney music, then you'll enjoy this (Little Mermaid, B&B and Lion King are in it that I remember). It's in 3D, and it's certainly something worth doing - with or without kids.