I don't know that I'd recommend Munchkin for anyone other than big fans of whatever genre you're playing, and even then, it's hard for me to recommend it.
Munchkin is essentially a comedy game that consists of a ton of inside jokes. Unless you're very familiar with a particular genre, you won't get most of the jokes. That wouldn't be so bad if the game itself was really good, but it's not. It consists of each player, on his turn, playing stuff to benefit himself, and then drawing a monster card, and fighting it. The other players will then play cards to make the acting player fail at fighting. If you win, you gain a level and some treasure. First person to level 10 wins.
What winds up happening is that by the end of the game, everyone's got numerous ways to screw over other players, so the endgame drags out forever. It really overstays its welcome, and most every time I've played it, I spend the second hour (third hour too, sometimes) just wishing it would end.
Personally, for a group of grandparents, parents, and kids, I'd recommend something like Bohnanza
. Both are inexpensive, easy to learn, have a good amount of depth, are fun, and their themes won't get in the way of anyone enjoying them.
Bohnanza is a game about bean farming and trading. Each player has a hand of cards depicting different beans which they'll trade with other players in order to get the most of a single kind planted in each of their two fields, which they can then sell for money. It's all about negotiating good trades, so there's a lot of conversation, and it's very easy to pick up on.
Carcassonne is about creating roads, farms, and cities by connecting tiles. Think dominoes, except instead of connecting numbers, you're connecting landscape features. As you lay features, you can claim them, and when they're finished, any that you've claimed score you points. It's nice because the rules are simple, and the older folks can help the younger ones figure out where to place the tiles. It's almost like a group jigsaw puzzle.
Either one of those I would think would work better than Munchkin from what you've described. There are some people who really love Munchkin, but I suspect even they wouldn't say it's a fantastic game to play with your parents (unless they're actually Cthulhu or D&D nuts).