I concur with isn31c to not add Windsor Castle with your visit to Highclere. The public transportation logistics to do both in the same day will eat up more time than desired.
Based on what is described for the Highclere Castle Festival, your family could easily spend 2-3 hours with the various activities and letting the kids run around on the lawn and enjoying the music.
I visited Highclere on Thu 24 Aug 2023 with an early afternoon entry to the house. While there wasn't a queue to enter the house at the time, the rooms were still fairly crowded. While the outside temp was comfortable, when you have more bodies in smaller spaces (hallways between rooms without cross ventilation), I found the inside house temp to be a little uncomfortable. I took a break between the house tour and the Egyptian exhibit in the basement rooms. The Egyptian exhibit in the basement rooms was worse as far as ventilation (even with fans turned on) and clamminess with the number of bodies in the rooms so I hurried through more than what I would have wanted to otherwise.
My recommendation is if there is a strong interest in Egyptology, do that part of the visit first when fewer people are there. Then do the upstairs house tour. The outdoor temps in October are likely to be lower than what I experienced in Aug, so it's likely to be a different situation; however I hope that sharing my experience will help make your visit more enjoyable. If the interests of the two adults (I'm assuming you have two adults with the two youngsters) differ, you might want to split up with one of you minding both kids while the other does the house or Egyptian exhibits and vice versa.
I wish I could attend the Highclere Castle Festival -- the list of activities sound very interesting!
