Kindergarten? So, 5 or 6 by next year?
I would certainly look at the Weehoo tandem, especially if you can find a used one, but it may not fit your needs for long.
There are regular triple tandems, that if done right could expand with the kids needs. They can even get couplers.
I think the Bike Friday Tandem will naturally fit people of all sizes, but I think only a 2 seat version.
Co-Motion in Eugene makes custom Triples. Probably not cheap, but worth exploring. You could probably get S&S couplers to haul around easier.
Man, this would have been a steal, even if not the classiest ride.
https://www.ebay.com/itm/MICARGI-Tri...-/333355188562
But, if you are giving yourself, say 6 months, keep your eyes open and something better might pop up.
No matter your solution, by age 8 or so, you may well have the kids riding on their own bikes with you in traffic, if not sooner.