It's really cool that your folks went out and got bikes. Ok, Fred Meyers, I know, not the kind of care you're going to get from your local bike shop - but there's probably a decent return policy, too. If you get up there and the fit is completely awful, they could take them back and get something that won't cause injuries later. Alternately, you could buy your parents professional fittings at a bike store in OR, in case you won't be there for a while.

As for motivation, you could call your mom, tell her you're really glad that she's doing something proactive - getting bikes - and that you just worry. Then launch into a conversation about all the fun of cycling. Make sure she feels good about this step. Compare notes on your bike routes. You could maybe send them some cool jerseys (or maybe just nice wicking tops, if they don't want to look too super-cycle-y). Try to bring (or borrow) a bike when you go up there and then ride with your folks. I'm sure they'll be tickled to go out with you.