Yep - you'll never learn so many new things as on your first few shoots
The 24-105 at the 105 end (ish) would probably be OK. I use the 70-200 almost excludively for these kinds of shoots, but on a 1Ds3, which of course is full frame.
VAL and a diffuser will make a BIG difference. Even a couple of VALs (parents are good for that). Also - if there aren't good locations around the parents homes, take them to a local park; getting some colourful foliage in the background will make the job a lot easier.
You need triggers that will enable HSS mode on the 580EX II, otherwise you'll be limited to X-Sync speed of around 1/250th - and you won't be able to get open apertures for better bokeh. Flashes + brolly/softbox attached to a monopod (or tripod) work well (that's what I use). Natural light sucks - especially if you're shooting in the open in the middle of the day
It'll cost you a lot more not to sign up in the long run - it's hands-down the best video training out there. It's only USD $24.95 for a month (you can cancel after that) - so I'd suggest signing up for a month and really really really getting stuck into those videos I mentioned. STRONGLY recommended.