Hi Amber,
Thanks so much for your question! I think it is really wonderful that you and your brother are taking your families on vacation in memory of your Mom. Disney is a wonderful place to make priceless memories, so this trip will be extra special. Make sure you take advantage of the
PhotoPass photographers so you can get some great shots with your entire group.
Since you will be visiting in August, you can plan on very hot and humid weather. Children this age tend to "melt down" fairly quickly, and often would prefer to spend time at the pool. For this reason I would be sure to choose a resort with a great pool. One of the best pools I've seen for small children is at
Disney's Caribbean Beach Resort, they have a toddler splash-ground that is simply amazing!
I would suggest that you spend at least two days at the Magic Kingdom, and get there early! This is the best way to avoid the heat and the crowds as much as possible. You can head back to the resort around lunch for a break and a swim, and then head back for a little more fun once the hot afternoon sun has passed.
If you plan on visiting the other three parks as well, I would schedule at least one day at each. Some can be done quicker than others, but traveling in a large group (with small children) can slow things down a bit!
I hope this helps, be sure to come back with any other questions you might have!