I much prefer to start packing as early as possible so that you know you won't have to scramble at the last minute to get everything in boxes. Since you have a little one, you won't be able to spend hours each day packing so doing a little bit at a time will be easier for you.
The best advice I can give you is: Purge as you pack! Nothing goes in a box unless you know you will be using it at your new house. It will probably cut down considerably on the amount of "stuff" you have to pack (and unpack). If you are not having any more children, you can get rid of the baby items your son can no longer use. If you may have more kids, get some plastic storage bins to organize the clothing and/or toys he has already outgrown so when you get to your new house you just have to put it in the garage or basement. A lot of people seem to gather a lot of unneeded kitchen gadgets, utensils, and lots of books even that they really no longer need or want so now is a great time to get rid of those items.
As the others have said, label the boxes on more than one side, and be specific. A box that says "Kitchen" is a little helpful, but if it says, "Kitchen: Glasses/Stemware/Plates" that is more helpful because you may need to get into that box right away vs the "Kitchen: Bakeware" box that you probably won't need to access for a few days. It will be helpful on that first night or the first few days after.
Don't overpack your boxes, either. As you fill them, lift them periodically so you know when to stop filling them. Make sure YOU can lift them all yourself...because if your husband isn't home, you will be out of luck if you need to dig through boxes!
Focus on setting up your sons room first (assuming he sleeps in his own room).
I have always packed a box just for the cleaning items/tool essentials I will need right away. I put that in the truck or car last so it is the first box that I unload. I always wipe down surfaces & the inside of cabinets as I put things away.
I don't have cats so I can't help too much with how to make their transition easiest...I guess if you have a neighbor or friend that could watch them at their house on the actual moving day that would be best. You will have doors open constantly so your cats could easily escape (assuming they are indoor cats only). Otherwise, I would probably lock them in a bedroom or bathroom with their litter box, food, & water. They will probably be stressed for a few days no matter what.
I happen to LOVE moving! I love the feeling of a new environment, fresh start, new decorating, etc...so good luck to your family!
