L.,
Last August my husband and I took our (then) 2 yr, 4yr, and 7yr old children to Disney. We are planning to return this June! I have spent considerable amount of time researching and planning for both trips, so if you have other specific questions feel free to email me directly!
here are my suggestions:
Instead of those kid leashes or other safety devices, we had personalized temporary tattoos made (safetytat.com). The tattoos said, If lost please call... and I put my cell phone number on it. That way my kids couldn't remove a bracelet (or whatever else contained important info) and we didn't have to worry about a stranger calling them by name and confusing them. The kids were excited to put them on and they made us feel better. We didn't lose anyone... but also took the time before to explain to them what to do if we were separated (find a cast member and wait for us to find them).
take a photo every morning of your kid and your parking section. If you lose your child, you won't have to try to remember what he/she was wearing that day. And... you'd be surprised to find how hard it is to remember if you parked in Goofy, Grumpy, or Dopey, Aisle F =)
Bring your own stroller. The ones you can rent are hard plastic strollers that don't recline and $15 a day. Additionally, you can't leave the parks with them so you have to get back to your car on foot!
Book character dining as early as you can. Disney allows bookings 90 days out now (used to be 180). I would suggest Chef Mickeys. All three of my kids L-O-V-E-D it. We purchased autograph books for a couple of bucks prior to our trip and avoided the over-priced ones you can buy there. Bring a pen (or marker) that is oversized so it will be easier for the characters to use.
Kids under 3 eat free.... even at the character meals!
Bring water and snacks. Lines can get long and the last thing you want is a hunger-induced melt down.
Definitely break for mid-day naps. We entered the parks as soon as they opened. We stayed until lunch time when crowds began to thicken. After lunch in our hotel followed by a nap for my younger two and a swim for the oldest, we returned to a less crowded, cooler evening with well-rested kids! We were even able to stay for fireworks. The kids really need the down time and you might too!
There are tons of helpful planning websites out there. Last year I subscribed to touringplans.com and this year I'm trying tourguidemike.com. They both give you info on the crowd levels at each park for any given day. They also have tools to help you plot a path to avoid long waits. I followed the touring plans created for my trip and the longest line I waited in was 5 minutes for the Magic Carpet ride at Magic Kingdom. Really... 5 minutes and only one ride!!!! (**which reminds me, take advantage of the fast passes for the popular rides when available). Allears.net, mousesavers.com, wdwinfo.com, and ourlaughingplace.com are some of the useful sites.
We didn't buy the park hoppers and I'm so glad we didn't. With kids so young, there was no need for hopping! One park per day was more than enough... even with a mid-day break!
I had a very slim fanny pack (the "presto"-style from Gwen Stefani's line, Harajuku Lovers)that held my id, park tickets, money, keys, and cell phone. It was small and thin, so I barely noticed it and it didn't interfere with the restraints on rides. I also had a cheap (free from hospital)diaper bag with an extra set of clothes, bandaids, neosporin, moleskin (for blisters), ponchos, etc. I carried my camera with me and I didn't put anything of value in the bag, so I was able to leave it with the stroller when we went on rides without worrying about someone stealing it!