Hello! First post…already excited to plan our first family trip even though we are 3ish years out (tentatively planning for early 2021). Will be myself, DH, a 5 and 3 year old. Looking to do 6-8 days where we can see a lot without stressing about cramming too much into a few days. I’ve been mentally planning this trip in my head since I was a kid in the 80s so while I know this won’t be our one and done trip I am excited to hear others tips to make this first one magical but not stress myself out! 
unfortunately due to work obligations Sept through November is out so I’d though Jan/Feb might be a good option…plus trying to avoid the heat of summer. I’m sure there are a lot of factors I’m missing so looking to the group to share their awesome insight. Thanks so much in advance!
As you might have read already on other posts and web sites, it is getting to the point where WDW has very few slow days. And with Star Wars land and all the other new attractions planned in the next few years, and the upcoming 50 year anniversary, the consciences is that WDW will not have slow days. I think that if you make plans to suit your needs (Work, weather, budget, convenience) followed up with ADR, FP+, and a good Touring Plan, everything will still be fun. And you will get to experience everything you want.
Looks like January-February will be your ideal time. In those months, times to avoid will be the RunDisney weekends in January, MLK weekend in January, and President’s Day weekend in February.
Agreed that with all the changes coming ahead and Disney’s constant tweaking to events and incentives, trying to decide what low crowds may look like in 3 years is hard. That being said, with your desire to avoid summer and inability to do fall, you are looking at a window of December through May.
I would suggest early May. Spring break is over, summer vacation has not yet begun. It is warm enough to feel like Florida, without the high heat of summer (therefore swimming is a huge plus). Just a thought. and as @donald263 said - with enough planning and a good Touring Plan, you can have fun whenever you go.
I went the Tuesday-Saturday directly following Easter Sunday 2017 and the crowds at MK were reasonable. It was in the 80s-low 90s every day.
I prefer going for the Christmas Holiday decorations. If you can go in Dec before approx 20 Dec, crowds have been manageable.
All things are possible with a good touring plan!
Especially with the little ones…you might consider going in early December to see the amazing Christmas decorations…and that period just after Thanksgiving has been a time for lower crowds. We went the first week in December in 2015 and loved it. It will be hard to predict for 2020 and 2021, with SW set to open in 2020 and all the 50th Anniversary goings-on in 2021. At least you have lots of time to judge how things will be shaping up.
I hadn’t thought of May. Sometimes my work can be busy then, but if I’m planning this far out, I can plan around that too! Its just Fall that is a real no-no given the volume of projects/programs. We are planning on driving, so that would help out the “what type of weather am I in for” factor as well coming from Southern Ontario, Jan/Feb can mean weather issues too…although not a deal breaker. Thanks again!
Since our children are grown and we don’t have to go in the summer anymore, we prefer either the week after Thanksgiving or 1st week of Dec for the holiday decorations and special activities, especially in Epcot. Your children should enjoy that. Also we enjoy the 1st week of May for the Flower & Garden event in Epcot. It’s a little warmer then, but no where near the heat of summer! Since you are obviously a good planner - 3 yrs out kudos to you! 
- you will have a great time and be able to work around the crowds any time you go. I might suggest starting to get Disney Gift Cards every month. It really helps the budget to get one every month or so before the trip. Make sure they don’t expire, but we’ve bought 2 or 3 yrs before our trips and they were still good. Don’t over stress and over plan - plan but leave room for surprises. Enjoy some of the Magic Moments you will surely have!
I love February at the D! Going next week.