With 4 kids, two of whom will be in car seats for the next decade almost, I would For Sure go for the minivan. That said, I'd trade in the highlander for a similar priced minivan, and keep the other car as is. It will take about a day of trying to vault two older kids into the way back (good luck with that if they are in car seats or boosters still too, lol, and you have to somehow get back there to buckle and unbuckle) before you are screaming for the minivan. Getting everyone in and out of that vecle will add time on either side of what already promises to be a super fun morning routine.
But, big but, what are your work hours? It's hugely unfair for two working parents with similar hours required and similar responsibilities, to not split daycare drop off/pick up. Hopefully you can arrange it so one of you does m/w/f and the other does t/r or something like that, rather than one doing mornings, one afternoons. But for me and my kid, when H and I were both working full time, doing that meant the kid had a seriously long day in a daycare and we weren't ok with that. That required me to go into work early, and pick up the kid at a reasonable time, while h dropped the kid off later and then worked later. YMMV. But I can certainly see why you would want two vehicles that can hold all the munchkins.
And even if it's legal to put a kid in the front seat (it is in my state if younger children are filling up all back seats) it's the kind of thing I consider acceptable in "emergency" situations, but not in a daily basis. I don't have any data to cite, it's just a gut feeling, so someone feel free to prove me wrong on the safety of that.