We've taken my grandson to Upper Pines several times, while he was still on a bottle. If you bottle-feed, I'd leave the milk, formula or whatever right in the front of the bear box for easy access at 3AM 8^).
My daughter once got up to refill a bottle and spilled a small amount on top of the bear box. At that time of night, she just ignored it. Later I heard a noise, and looked out, and on top of the bear box was a bear, happily lapping up the spill.
Just keep a trash bag for the diapers, and put it in the box.
Occasionally a bear will get pretty brave. When I was there in October, some folks in a nearby campsite were eating dinner at the table. A bear jumped on the table, grabbed their bag of chips, and ran off with it.
(the traps were set the next night...that's usually not tolerated behavior)
So keep the smaller kids close, for various reasons, but doing that, everything should be fine.
If you're going in early August, at 7AM on March 15 I'd suggest you have the website already open to the campground you want, credit card in hand, a site map handy, and have already decided on a favorite few spots or areas. If in later August, the same but on April 15. Ten minutes late and you probably will get a site, but maybe not, or not where you wanted.
Upper pines is nice too, and closer to the shuttle bus for any after-dark returns to camp. (so if you hit a snag getting in at North Pines, don't hesitate to take Upper Pines; frankly, I like it better, but they're both fine.
Gary
Yosemite Photo Galleries: http://www.pbase.com/roberthouse/yo