I have a four year old at home and his personal favorites that allow local multiplayer are: Rocket League (PS4), Towerfall (PS4), and Smash Brothers (Switch)
Best of luck through all this. He's only been in quarantine for about a week so I imagine we'll reach your point in a couple more weeks.
Thanks Tor. Definitely going to fire-up the PSstore. I'd appreciate more split screen suggestions.
It's looking more and more like we're going to get hit hard in America. Stay safe and stay home. Ironically I feel much safer here in the UAE than I would have if I was currently in Cleveland. And with the spring break incidences... ugh ignorance. I'm kinda relieved I delayed quitting my international job last year.
When I have access to my PS4 again I'll check around for what other PSN games I've got that support split screen or otherwise local multiplayer. He's also a big fan of those Overcooked games but it's not something he's actually played successfully while the other games I listed he's gotten surprisingly decent at.
I work in academia and my wife is in grade school education so they've been home from work for a while now compared to the rest of the US. I think the numbers are going to get extremely high, but I'm hopeful that most people I know have been taking this seriously.
Edit: He's also enjoyed LBP4 and Dreams, but I'd go for LBP4 over dreams since it's easier to get his head around a game with more consistent rules and feel. Also I think Dreams doesn't support local multiplayer yet but I might be wrong on that.