There can be snowstorms in Yellowstone even into June (I got caught in a particularly nasty storm there over Memorial Day weekend once), and by July it is super crowded.
There are only a few hotels and inns in the park itself (and these, as you said, would be difficult to get a summer reservation for), but there are many hotels, motels, inns, and cabins in the towns just outside Yellowstone. The most convenient of these is probably West Yellowstone, which is only about fifteen miles from the Grand Loop that runs through the park. Gardiner, Montana is also just outside the park--it's the northern gateway to the park, and is only a few miles from Mammoth Hot Springs. It's been a while since I've been to either of these towns, but both of them were, at one time, nice towns in their own right. West Yellowstone, from what I understand, has gotten increasingly commercial over the last few years, though. Cody, Wyoming is the closest town to the Eastern entrance, and is a nice, enjoyable Western town. They have a really good museum, some good food, and it's a pleasant drive into the park. But it's a bit too far away to be practical as a home base while exploring the park, imo.
I don't know if your friends are up for camping or not, but this is what I would consider the best bet. There are lots of campgrounds in the park. The last time I checked (and it's been a while) there were some spots that could be reserved, but many of them are first come, first served. They're pretty cheap--all of them are less than 20 bucks a night (unless you've got an RV), and some are as low as 12 bucks a night.
My personal favorite campground is probably Mammoth--it's big enough that you can generally get a spot, but not super crowded like the huge campgrounds at Grant and Bridge Bay. There is a service station and a small store so you can stock up on food, etc. There's a historic Inn (I've stayed there, too, but it's expensive and I kind of prefer camping), and of course the hot springs, which are a great attraction. There is also a ton of wildlife at Mammoth--or, at least, there has been every time I've been there--especially elk. Once when I was staying there I took an early morning walk, just before the sun was to come up, and was treated to a huge herd of elk, some bugling in the otherwise still mountain air. Damn that was a beautiful scene.
As for the tours--I haven't done a lot of formal tours there, but the rangers are very good and informative. At most locations they have a walkway that you guide yourself along, and they have maps/pamphlets with information as well as informative markers along the trail. So if the parents read English as well, they could conduct the self-guided tours fairly easily. (They might have Spanish language pamphlets available, too, but I don't know for sure.) That's not to say there are *no* guided tours, just that you don't need guided tours to have a good time in Yellowstone. There are a lot of great specialty tours that are guided by rangers--one of the best of these is the Norris Geyser Basin at night--but I don't know if they have any in Spanish. They also have special programs (Ranger talks, Video presentations, etc.) at some of the major locations (Mammoth, Old Faithful, etc.). Again, even if none of these are accessible b/c the kids don't speak English, a great time can still be had.
The NPS Yellostone page has a lot of information:
http://www.nps.gov/yell/. Oh, and one more thing: The animals are wild and they are dangerous. I know that may sound obvious, but every summer there are people who think they can pet the buffalo. If your friends aren't used to the wilderness, please impress upon them a healthy respect (not fear, but respect) for the wildlife. :)
Anyway, I'm quite jealous of your friends, as Yellowstone is one of my favorite places on earth. If you've got other questions I'd be happy to tell you more, or to make specific recommendations, if you'd like. :D