Two days at Disney, followed by two days onsite at Universal. Universal is much smaller and getting place to place is easier and takes a lot less time. Going for the first time you may be surprised how quickly the miles at up at WDW. Nice to follow it up with two more relaxing days with short lines, a lot less walking and just more down time to relax and recover. Universal is much easier to navigate and citywalk is handy for dinner in the evenings. Staying onsite is worth the express pass.
I’m also pro hoppers. One park at opening, ending with lunch (offsite if you aren’t staying onsite because it is cheaper and you have more options, even some delivery to your hotel might be a good choice) around 12. Downtime back at the hotel (are you onsite? If not is your hotel nearby and have a pool?) to recharge, nap or lay by the pool for a couple of hours and then into the second park from 330-9.
When we hop, we make a clear plan of what we want to do and then let our interests guide the rest. What you don’t want to do is hop with zero plans and just find yourself wandering. Make use of your fast passes, pick a place to eat in advance, and be familiar with layout of each park and the areas you want to explore.
I’m a magic kingdom girl myself and could be happy in just that park for multiple days. If you do decide to hop just factor in travel time and how your three year old will do with all the changes.