For little kids Magic Kingdom tends to be their favorite and there's the most for them to do there so I'd go 2 days at that park and then 1 day at each of the other three parks (or do one day at each and sneak a waterpark in depending on if they're open on the dates you are going).

Just a general tip, though, if you are staying at a WDW resort hotel or timeshare, you can get in to each park 30 minutes early and you'll be able to do a lot more by taking advantage of that first 30 minutes with short lines at the top attractions. If you're staying off park, all the Disney hotel guests will be on park already for 30 minutes before you are allowed in so your first hour on the park is not as productive as it used to be by getting there for "rope drop."

Magic Kingdom: It's basically Disneyland with more good places to eat and shows, but fewer good rides (especially for little littles). You need to book restaurants on park 60 days in advance using the app and your park passes. Also, there's the Mine train ride in lieu of Matterhorn and space mountain is not nearly as good for some odd reason, but splash mountain and thunder mountain are essentially the same.

Hollywood Studios: This park is a lot of fun, but it's kind of a miss for little little kids. The park in general is almost always slammed right now because of two new lands (star wars land and toy story land), so go here mid-week if possible. Rise of the resistance is one of the better rides/experiences I've ever done at a theme park as it has a lot of really cool elements to it, but it will be a commitment of time for you and the older one or two kids. Definitely child swap it. The slinky roller coaster is fun enough, but the lines get insanely long for it so get it done early in the day if possible. You and your spouse should each do the single rider line for Rock-N-Roller Coaster while the littles and the other spouse hit shows (there's a cars show right next to it that's cool for little kids). Also, two of my favorite restaurants at WDW are at Hollywood Studios but both are kind of spendy: Hollywood and Vine (a fancy Buffet with really good food for Mom and Dad and the standard fare for kiddoes), and the Brown Derby (nice restaurant, really good food, probably a bit of a stretch for small kids though).

Animal Kingdom: It's not a big park so one day is fine, but the avatar flight ride line gets ridiculously long almost immediately and it rarely comes down in time over the day so it's best to get that over with right out of the gate if you want to do it (with small kids you might just want to pass on the avatar land altogether and enjoy everything else). Then hit up the safari ride because the line to that gets really long too, but typically not until late morning. After that work your way around. The lion king show is worth catching, but arrive 15-20 mins before showtime.

Epcot: Get in the virtual cue for Cosmic Rewind as soon as it opens using the app (I think it's 7am), and then use child swap to take whomever is 8 on it twice so both parents can go (don't waste your time with Ratatouie (Sp?). This is the largest park and its tough to do in one day if you want to spend much time doing the tour of nations around the lake. Littles love the frozen boat ride, but that line gets really crazy really quickly so get it over with early in the day. The evening show is decent (water, music, movies on water, fireworks) and it's easy to watch from basically anywhere around the lake so you don't have to find a spot an hour before and try to keep the littles still like with fantasia or the castle shows at magic kingdom (although keeping them there until dark can be tricky).