I take all my gear + little kids. I have a 4 year old and a 1 year old.
While walking around I hang my camera bag off the back handles of the stroller. That helps a lot in terms of carrying stuff around all day. Plus I can just grab it quick off the handle bars and take it with me into a shop or a ride (for the record, we have a BOB stroller, so it can handle the extra weight like a champ). DH and I will also take turns carrying the bag if necessary.
Ditto - except that I don't get to pawn my bag off on my wife.

(Except for rare times when we need to leave the stroller and I'm off doing something like changing a diaper.) Hanging the bag off the stroller is very handy. When I am solo and carrying the bag the whole time, it gets a lot heavier! I've got a 5-year-old and 1-year-old, for reference.
Straps help, too. I have an Op/Tech SOS Strap on my camera bag, which is wider and stretchier than the original one. If you (the OP) are still using the strap that came with the camera, you may want to invest $20 or so into an Op/Tech Pro strap - this is a big improvement over the OEM strap. You'll see the Photopass guys using them sometimes. I often see DSLR (and regular camera) users at Disney carrying their cameras by holding them by the neck strap - almost certainly because those straps are killing them! The BlackRapid R-Straps are picking up popularity but are very expensive (almost $60!)
In terms of gear - I'd say, bring your best stuff. Your fast-focusing DSLR body, your sharpest lenses - you don't want to be missing shots because the D50 can't keep up with the D90 (or have the low-light ability) or to think that your photos are worse than they could be if you used another lens that you left behind.
Some people carry around the external flash a lot... but I haven't used mine a single time on my current trip (which is on day 11 now.) There's only been once or twice that I would have wanted it (for fill light or off-camera use), and that's mainly for off-camera stuff. Your D90 should do well enough in low light to not need it too much.
One other thing I do is use a lens pouch that attaches to my belt to hold one or two (small) of my often-used lenses. This helps distribute the weight. I have two but I don't think my wife will be seen with me if I wear both, so I usually just have one.

Of course, women don't always have such an option - I frequently lament that my wife has limited carrying ability since women's pants so often don't have pockets or get worn with a belt.