DO NOT TRY TO PAT THE BISON!!!
Sounds like stupid advice, right? Like, yeah, who is dumb enough to try to pat a buffalo? Well, every year, some doofus thinks "Oh, I'll just pat the little one, its so cute!" Trust me, you don't want a mother buffalo bearing down on you, so stay in your cars when you're watching the animals - they are NOT tame and they are not in YOUR environment, you are in THEIRS. Respect that as rule #1 and you're more likely to make it home alive.
The second part to rune #1 is DO NOT FEED THE ANIMALS! They get along fine without smores and cookies and most of them would rather eat you.
OK, now that the important stuff is out of the way, here are some suggestions:
1.) Dress comfortably. It gets hot during the day and cool at night here in the rockies.
2.) Provisions. What with herds of bison, elk, wolves etc. traffic can move fairly slowly when you're traveling the loop roads. Be spart and pack a cooler before you head off sightseeing every morning. Cold drinks and snacks should be standard, but packing a picnic lunch is a great way to enjoy the park and avoid long lines at overpriced restaurants. (There are picnic areas all over the park.) Its also a great way to let the kids stretch their legs after lots of riding.
3.) Make reservations NOW. Book your reservations well in advance. If you wait until the last minute you may very well be looking all night for a place to rest your head. When my husband and I went, we were on a tight budget, so we stayed in one of the Old Faithful Frontier Cabins. They are right behind Old Faithful Lodge and are fully equipped with beds & baths. (They are pet friendly too!) We only paid $85/night there so it was an absolute steal! We had a perfect view of Old Faithful from our window and really enjoyed our stay. You can see all the lodging available in the park itself and book reservations online at:
http://travelyellowstone.com/
Have a great vacation!