My MIL has a place up there, so we've done this before. There's enough to occupy them for a long weekend.
Let them spend a bunch of your monies on go-karts and mini golf and all that stuff at Funtrackers. That will occupy them happily for most of an afternoon. There's a little park across from that where, even if they don't like the outdoors, they can goof around in the creek and get soggy for an hour or so.
If you don't have dinners already planned, they'll probably like Farley's, as its target demographic is frat boys and other nascent adolescents, and you can get a good pizza and beer there. I think the Irish pub there closed down permanently (it was OK), but there are a couple decent beer joints. Rio Grande Grill had pretty good food and brews, but nothing particularly kid-attractive (I've conditioned my boys to like brewpubs, so they thought it was great).
I will say that the Log Cabin is awesome for breakfast, probably the best meal I've had in Ruidoso.
My boys have enjoyed the Billy the Kid stuff in Lincoln and the Smokey the Bear museum in Capitan, but at that age you might get more eye-rolling than you'd like, especially since it's a bit of a drive for a long weekend trip. There are some other museums around, but it's summer, so no enrichment, please.
I think it would be a great idea if the grandparents took the kids to the chuckwagon dinner and western show at the Flying J Ranch, while you and the wife go have drinks and dinner somewhere else and then shag in the hot tub (safety first) while they're gone.