My husband and I took our 8 year old niece, my mother-in-law, and brother-in-law to MK this past Monday. I was anxious about both the crowds and the temperature, but the day ended up being one of the best we have had in the MK.
We arrived about 20 minutes before Rope Drop. We are usually late arrivers,but we wanted to take advantage of the cooler temperatures. It was a lot of fun and a worthwhile experience. We meandered on Main Street for a while and headed up to Fantasyland for our niece. We also sent a "scout" out for FP to Big Thunder and Buzz Lightyear. We should have done this for Space Mountain as well since this had the longest waits during the day. Midday, Tomorrowland was the most challenging crowd wise, so I think you are smart to do those early. Honestly, I feel like I have seen crowds heavier and the lines longer in the Fall and Spring months. We had a really great day and just played it by ear as far as the direction we took.
Although I never really found the heat a problem since we had periodic showers through the day, we took a couple hours off to head over the Polynesian for leisurely cocktails and sushi. We went back for the evening hours. Finally, we watched the 10 pm fireworks from the beach at the Polynesian (they were as fantastic as you have heard). A relaxing visit all in all. I hope that yours goes well!