Appreciate comments from those who have experienced Columbus Day weekend. I expect more crowds than other parts of oct. But hopefully not on the order of spring break, Christmas etc? With young children and staying on site, can we survive by getting to mik early and staying until lunch time? Or us it just miserable?
Absolutely not Christmas crowds an, in my experience, not spring break crowds either. I called them "moderate plus", meaning there were plenty of people, parades and such will feel like an awful lot of shoulder-to-shoulder humanity, and in the peak of the day or if you happen to have the bad luck to be right where crowds are, you'll find plenty of long lines.
However, your plan for crowd avoidance is great. I'm not sure if literally, you are there just sat, sun, mon of Columbus Day weekend, or it is part of a longer stay.
Monday is actually a recommended day for MK because it closes early (7pm) for the Halloween party that night. Lots of people don't want to use up an MK day when they can't stay for night parade or fireworks, but that means it's very good in the daytime. We travel in Oct and love the daytime part of party days. If you want to see the fireworks, you can go a different night just for them, or watch from outside the park, or substitute night shows like Illuminations or Fantasmic in other parks instead.
Saturday and Sunday will be noticeably more crowded than Monday, but still not Vhristmas/spring break levels. The other parks don't have these wild swings from day to day and I'd still consider them moderate/moderate plus each day.
Have fun!!