Mario Kart, I feel, is plenty good enough with exclusively local splitscreen, especially at its price point. I've considered it but will probably hold out for MK9 to go double.
ARMS I've considered, but I don't have it so was uncertain. It's at a decent price used, so it might be worth picking up two copies.
@JRPGfan - It's not just about them playing the games together, it's also about me playing with them. The point is just games that can be played together with two consoles, but preferably are family-friendly, because even though they're young, they'll also grow into them. Plus, I'm not sure where you're getting that the Pokemon titles aren't good for it... Pokemon in general is one of the best franchises out there for that, and Sword/Shield is going to have raid battles to further increase the interactive play potential. If it's a question of whether they'd be capable of playing them, I guess you're just inexperienced with young children, because my kids are by no means the most advanced gamer kids I know and still have no problems.