As the father of twins, and a frequent birthday & child photographer, there are something things to keep in mind.
1. Kids move fast. Faster than you.
2. You'll miss quite a bit no matter what happens.
3. Changing lenses won't do you any good, as it's too slow. You can do this between sets (Play -> Cake -> Presents), but don't expect to do it much.
4. A flash is more trouble than it's worth.
When I shoot these events, I shoot all the candid's with my 5DIII and the 70-200 F/2.8. I leave it in Aperture mode at F/2.8, enable auto-ISO up to 25k, shoot available light, and set AFS focus. Having a long zoom is a huge help, as I can shoot far more of those fast moving little kids.
When it's time for the birthday cake / singing, I generally use the same lens. I am mostly isolating the kids and not getting group shots. When it comes time to blow out the candles, I bust as many shots as possible to try get one with the right expression.
If I need group shots, I use the 24-105 F/4L. If I had the 24-70 I would probably use that, but that extra range is usually more important.
In terms of flash photography, I find it's horrible for candid kid shots. Kids won't look at you because they don't want their eyes to hurt, the flash recycle time is too slow, and it puts too many limits on shutter speed (unless you use hi-speed sync). The more you can blend in and be ignored by the kids the better, and a flash will not let you blend in at all.
- Shoot the kids from their eye level or slightly below. This means you're lying down or sitting down. NOT standing. This makes a huge difference, but will also slow you down making the range of the 70-200 even more important.
- Relax. Be friendly. Enjoy. Engage. It's you're a stressed out jerk, the kids will pick up on it.
- Print a book after, if your budget allows for it. With Lightroom this is trivially easy.
- Don't rent lenses. Don't buy new gear. Don't stress over things. Don't watch the clock.
- Like a wedding shoot, have the parents give you a list of group shots they want. Insist on this, and then get those shots.
- Bring bunnies. :-)
Note: I have never, ever, gotten a great candid shot of two kids at once - and I have twins of my own and lots of time to practice! You cannot shoot twins at F/2.8 as the DOF is too shallow, which means that great bokeh is out. They never, ever, smile at exactly the same time and look in the same direction. I can get two smiles, but they're looking different places. I can get both looking the same way, but only one smile. It's photographically very frustrating. Occasionally I'll get everything right and realize my camera was still at F/2.8 and one of them is slightly out of focus. Or I'll be at F/11 and the image is so cluttered with background material that it's just too distracting. For shooting group shots, I have found I need to stage it rather than getting candids. With a pair of 4 year olds, this should be pretty easy.