I really like the Flipside series for two reasons. First, the main compartment opens against your back, making casual unzip-and-grab theft more difficult. Second, I find it makes lens changes easier - you slip it off the shoulders and pull it to the front, and the bag becomes a platform for the lens change. Alternatively, if you do want to remove it like a traditional backpack, at least when you set it down, the side that rests on the ground isn't the side that goes against your clothes.
One more suggestion, along the lines of one lens per outing. I'm not sure that works - at the zoo or Sea World, you'll likely need the 200/300mm end (animals in enclosures, kids on rides) and the wide angle for closer family shots. The 'one lens' can work if you pick one or the other type of shot (or in my case, where that one lens is the 28-300L). One thing that might work, though, is to take just the long lens, and use the S100 for close/wide shots.
What I've found on many trips (not considering the superzoom) is that two lenses works well, e.g. standard zoom + telezoom for daytime outing, switch to standard zoom + fast prime for evening-to-night outing, etc. The issue then is how to carry the spare lens. Personally, I've used several solutions. My camera is always carried on a BlackRapid strap. I sometimes also carry the Toploader with a Lens Case attached, over the other shoulder, straps crossing on my chest - that holds the spare lens, but is a bit bulky with excess empty space. Alternatively, I use a Lowepro Deluxe Technical Belt - a bit Batman-esque, granted, but functional. BR strap with camera hanging on the right side, left side has a Lowepro Lens Case sized for the larger of the pair (meaning it will hold the smaller one, too), a water bottle holder, cell phone holder, etc. I've also done two lens cases, and it's comfortable.
Back to the 70-200/4 IS vs. 70-300L issue, having just been to an amusement park with the kids, 300mm on FF was very nice, and 200mm would have not been as effective. Likewise, as a former SD resident and Zoo member, the longer the better. There's a 10 oz. difference in weight. I know that adds up, but selecting a bag with two shoulder straps and ideally a hip belt will more than make up for 10 oz. in better weight distribution. Having a good pack makes weight less of a concern. What is a concern, to me at least, is size - I'd find the 1" shorter (retracted) length of the 70-300L to be an advantage that outweighs the weight gain - shorter length means it fits 'upright' (vertical) in a bag, taking up much less space than a lens that needs to lay on its side, possibly meaning you get a smaller bag.